arm: switch ARM/Thumb decoding with `t`

`c` could not carve Thumb code. Thumb isn't a property of the bytes — it's a
mode the CPU is in — so a raw image gives IDA nothing to detect: at a Thumb entry
point it decodes 16-bit instructions as 32-bit ARM and produces confident
nonsense. experiments/fibonacci.bin starts with `08 b5` = push {r3,lr}, which
IDA reads as SVCLT 0xBF00.

`t` on the listing switches the mode at the cursor and disassembles in it:

  Thumb @ 0x0 (segment set to 32-bit; Thumb needs ARM32) — 10 instructions
  0x0 PUSH {R3,LR}   0x2 MOVS R1, #0   0x4 MOV R4, R0   0x6 BL unk_E3C

Setting the T segment register is only half of it. Thumb does not exist in
AArch64, and a headerless blob loaded with -parm comes up 64-bit, so T alone
changes nothing and looks broken — I watched exactly that happen while probing
the API. Asking for Thumb IS asking for ARM32, so set_thumb forces the segment
to 32-bit and says so rather than doing it silently.

It also has to del_items over the range first: the bytes are currently decoded
in the old mode, and leaving that item defined pins the wrong instruction length
so the new mode has nothing to apply to.

Implemented as a `thumb` kind in the existing edit-item flow, so it inherits the
shared reload — same cache bump, same ViewAnchor restore, same status flash. It
switches AND disassembles, because flipping T and leaving the bytes undefined
shows you nothing and reading the code was the point.

tests: new tests/test_thumb_ui.py (8) driving the real Thumb binary — `c` alone
does NOT produce the prologue, `t` does, the instructions are 16-bit wide (in ARM
mode those three rows would be one 4-byte instruction), the run continues, the
status explains the 32-bit forcing, and `t` toggles back. Deletes the .i64 first,
because T and the segment's addressing mode are saved in it and a stale database
would answer the question for us.

209/0 scenarios, 26/0 blob, 8/0 thumb.

1 files changed, 8 insertions, 0 deletions

diff --git a/idatui/domain.py b/idatui/domain.py
index 606cf42..8787431 100644
--- a/idatui/domain.py
+++ b/idatui/domain.py
@@ -1373,6 +1373,14 @@ class Program:
         if res.get("error"):
             raise IDAToolError("define_code", f"@ {ea:#x}: {res['error']}")
 
+    def set_thumb(self, ea: int, mode: str = "toggle") -> dict:
+        """Switch ARM/Thumb decoding at ``ea``. Returns the resulting state."""
+        r = self.client.call("set_thumb", addr=hex(ea), mode=mode)
+        if not isinstance(r, dict) or r.get("error"):
+            raise IDAToolError("set_thumb",
+                               f"@ {ea:#x}: {(r or {}).get('error', 'failed')}")
+        return r
+
     def define_code_run(self, ea: int, limit: int = 20000) -> dict:
         """Disassemble consecutively from ``ea`` until something stops it.