trace: memory at time T — stack in the dock, live bytes in hex

M2. Trace.memory(addr, length, idx) reconstructs what memory held at a moment,
returning the bytes AND a per-byte "known" mask. The mask is the point: a trace
knows what it observed and nothing else, so a byte nobody read or wrote is
genuinely unknown and must not be drawn as zero. That distinction is the whole
reason to read memory from a trace instead of the database — the database has
the file's bytes, the trace has what was actually there.

Reads count as evidence, not just writes: an instruction reading a byte reveals
what it held then.

Indexed by ADDRESS (sorted once, bisect per query), because the question is
"what was in this window at time t" and the accesses that matter are the few
touching that window, not the tens of thousands in the trace.

Where the memory actually is: measured, 0% of accesses in either real trace fall
inside the image — every one is stack or heap. So the primary view is the STACK,
in the dock, anchored at SP:

  stack (rsp)
  ▸7ffff6f99470 ????????????????
   7ffff6f99478 00007ffff6fb0b00
   7ffff6f99488 00007ffff6fa94e5

The hex view overlays trace bytes on the file's contents (green = the trace saw
this byte at this timestamp, grey = still the file's idea). Correct, and it will
matter for a program that writes globals, but on these traces it shows nothing —
which is why the stack pane is the deliverable and not a nice-to-have.

One bug the work surfaced: MemOp.addr was having the image slide applied to it,
which is nonsense for a stack address — it produced -0xc838. The slide relocates
the IMAGE; stack and heap have no database counterpart. Memory op addresses now
stay in trace space, and memory_raw() queries there, while memory() takes
database addresses for the hex view.

tests: +9 model (35) covering the known-mask, reads-as-evidence, partial
coverage and the writers/accessors queries; +1 differential (12) checking
reconstructed memory state against Tenet's own get_memory at sampled timestamps;
+5 UI (30) for the stack pane — present, anchored at SP, marks unseen bytes,
follows time. 212/0 scenarios.

3 files changed, 98 insertions, 2 deletions

diff --git a/tests/test_trace.py b/tests/test_trace.py
index 2803a11..58c9d81 100644
--- a/tests/test_trace.py
+++ b/tests/test_trace.py
@@ -87,6 +87,43 @@ def main() -> int:
               [o.write for o in t.memory_ops(4)] == [False])
         check("an instruction with no memory has none", t.memory_ops(2) == [])
 
+        # -- memory state at a timestamp ------------------------------------ #
+        # The trace wrote 2a000000 at 0xff4 (t=3) and read it back (t=4); it
+        # pushed/popped 8 zero bytes at 0xff8 (t=1 write, t=5 read).
+        d, k = t.memory(0xff4, 4, 3)
+        check("memory reflects a write as of that timestamp",
+              d == bytes.fromhex("2a000000") and k == b"\x01" * 4, f"{d.hex()} {k.hex()}")
+        d, k = t.memory(0xff4, 4, 2)
+        check("and does NOT reflect it before the write happened",
+              k == b"\x00" * 4, f"{d.hex()} known={k.hex()}")
+
+        # A trace only knows what it saw. A byte nobody touched is unknown, and
+        # must not be reported as zero — that distinction is the entire reason
+        # to read memory from a trace instead of from the database.
+        d, k = t.memory(0x5000, 4, t.length - 1)
+        check("untouched memory is unknown, not zero",
+              k == b"\x00" * 4 and d == b"\x00" * 4, f"known={k.hex()}")
+        # 0xff2..0xff3 was never touched; 0xff4..0xff7 came from the write at
+        # t=3 and 0xff8..0xff9 from the push at t=1 — a window can be knowable
+        # from several accesses at different times, which is what makes this
+        # worth a mask rather than a flag.
+        d, k = t.memory(0xff2, 8, 4)
+        check("a partially-covered window marks which bytes are known",
+              k == bytes([0, 0, 1, 1, 1, 1, 1, 1]), f"known={k.hex()}")
+
+        # Reads are evidence too: an instruction reading a byte reveals what it
+        # held at that moment.
+        d, k = t.memory(0xff8, 8, 5)
+        check("a read reveals memory contents",
+              k == b"\x01" * 8, f"known={k.hex()}")
+
+        check("memory_writes lists only the writers",
+              t.memory_writes(0xff4, 4) == [3], f"{t.memory_writes(0xff4, 4)}")
+        check("memory_accesses includes the readers",
+              t.memory_accesses(0xff4, 4) == [3, 4], f"{t.memory_accesses(0xff4, 4)}")
+        check("a never-touched range has no accesses",
+              t.memory_accesses(0x5000, 16) == [])
+
         # -- execution queries: what painting is built on -------------------- #
         check("executions lists every timestamp for an address",
               list(t.executions(0x401000)) == [0, 6], f"{list(t.executions(0x401000))}")
@@ -119,8 +156,12 @@ def main() -> int:
         check("addresses come back in database terms",
               t.ip(0) == 0x1000 and list(t.executions(0x1000)) == [0, 6],
               f"{t.ip(0):#x}")
-        check("memory addresses are slid too",
-              t.memory_ops(3)[0].addr == 0xff4 + slide)
+        # Memory addresses are NOT slid: the slide relocates the image, and
+        # these are overwhelmingly stack/heap addresses with no database
+        # counterpart — sliding a stack pointer by the image delta produced a
+        # negative address in testing.
+        check("memory op addresses stay in trace space",
+              t.memory_ops(3)[0].addr == 0xff4, f"{t.memory_ops(3)[0].addr:#x}")
         check("raw_ip still gives the traced address",
               t.raw_ip(0) == 0x401000)
 
diff --git a/tests/test_trace_ui.py b/tests/test_trace_ui.py
index 5d620a9..b672143 100644
--- a/tests/test_trace_ui.py
+++ b/tests/test_trace_ui.py
@@ -141,6 +141,40 @@ async def run() -> int:
                       app._t == ret, f"{i} -> {app._t}, expected {ret}")
                 check("which is further than a plain step", app._t > i + 1)
 
+            # -- memory at time T -------------------------------------------- #
+            # This is where a trace's memory actually lives: measured on two
+            # real traces, NONE of the accesses fell inside the image — every
+            # one was stack or heap. A memory view that could only address the
+            # image would have nothing to show.
+            dock = app.query_one(TraceDock)
+            app._seek(min(60, t.length - 1))
+            await pilot.pause(0.8)
+            stack = str(dock.query_one("#trace-stack", Static).render())
+            check("the dock shows the stack at this timestamp",
+                  "stack (" in stack and len(stack.splitlines()) > 4, stack[:60])
+            sp_name = next(r for r in ("rsp", "esp", "sp") if r in t.reg_at)
+            sp = t.register(sp_name, app._t)
+            check("anchored at the stack pointer",
+                  f"{sp:012x}" in stack, f"sp={sp:#x} / {stack[:80]}")
+
+            # A trace knows what it observed and nothing else. Unseen bytes are
+            # printed as '?', never as zeros — rendering them as zero would
+            # invent facts about memory nobody looked at.
+            data, known = t.memory_raw(sp, 8, app._t)
+            if not all(known):
+                check("memory the trace never saw is marked unknown",
+                      "?" in stack, stack[:80])
+            else:
+                check("known stack words are shown as values",
+                      any(c in "0123456789abcdef" for c in stack), stack[:60])
+
+            # Stepping must move the memory view with time.
+            before = stack
+            app._seek(min(80, t.length - 1))
+            await pilot.pause(0.8)
+            check("and it follows as you move through time",
+                  str(dock.query_one("#trace-stack", Static).render()) != before)
+
             # -- trails ------------------------------------------------------ #
             # Not "every address the trace ever touched": on a loop-heavy
             # program that's almost everything and says nothing. The last/next
diff --git a/tests/test_trace_vs_tenet.py b/tests/test_trace_vs_tenet.py
index 6c78c95..aa6002a 100644
--- a/tests/test_trace_vs_tenet.py
+++ b/tests/test_trace_vs_tenet.py
@@ -134,6 +134,27 @@ def compare(path, ref_cls, arch, dctx, samples=200):
     check(f"{name}: same execution timestamps for the hottest addresses",
           not ex_bad, f"{ex_bad[:3]}")
 
+    # Memory STATE at a timestamp — reconstructed from the deltas, which is the
+    # hard part and the whole point of reading memory from a trace.
+    mem_bad, mem_checked = [], 0
+    for i in idxs[::4]:
+        for op in ours.memory_ops(i)[:2]:
+            n = min(len(op.data), 8)
+            mine, known = ours.memory(op.addr, n, i)
+            ref = theirs.get_memory(op.addr, n, i)
+            if ref is None:
+                continue
+            refb = bytes(ref.data)
+            # Compare only the bytes we claim to know; the reference reports its
+            # own coverage separately and a byte neither has seen is not a
+            # disagreement.
+            for j in range(n):
+                if known[j] and refb[j:j + 1] and mine[j] != refb[j]:
+                    mem_bad.append((i, hex(op.addr + j), mine[j], refb[j]))
+            mem_checked += 1
+    check(f"{name}: memory state at a timestamp matches the reference",
+          not mem_bad, f"{mem_bad[:3]}")
+
     # Memory: the bytes an instruction touched, and which way.
     with_mem = [i for i in idxs if ours.memory_ops(i)][:40]
     mem_bad = []