projects phase 4: cross-binary back, linkage-guided pre-warm, project verbs

Three things, all following from phase 3 making cross-binary jumps ordinary.

**A cross-binary jump was a one-way door.** Nav history is per-binary, so
arriving in another binary — a project search hit, or now following an import
into the library that implements it — landed you in an empty history with nothing
to take you back. _switch_then_goto records the binary it came FROM, and
action_back falls through to that hop once local history is spent: Esc walks back
through the function you were in, then the binary you were in. Manual Ctrl+O
switching records nothing, because that isn't navigation.

**Pre-warm follows the linkage graph, not list order.** _prewarm_provider warms
the binary providing the most of this one's imports — where a follow is most
likely to go, so its startup is paid before you ask for it. "Next in the list"
would have been arbitrary; phase 3 gave us something better to ask.

pool.prewarm() refuses rather than making room. Evicting a binary the user
visited to speculatively load one they haven't is a straight downgrade, and it
throws away that binary's caches as well; at a tight budget pre-warm just does
nothing. The cost of a worker that doesn't exist yet can only be estimated, so it
uses the largest resident one (same program, different database) — and if that
estimate proves wrong, the speculative worker is the one evicted, never a chosen
one.

**Driving a project.** pane spawn --project FILE [--open BIN]; `binaries` lists
the inventory (active / resident / indexed / where Esc returns to) and `switch
{binary,addr?}` makes another active — with an address it takes the search-hit
path, so it records a hop. state gains `binary` and `hops`, which it should have
had the moment project mode existed.

Verified on real sessions: drive binaries/switch against an echo+cat project
pane; Esc crossing back from a switch; and prewarm on echo+libc picking libc
(provider of echo's imports) and warming it after an evict.

tests: +5 pool (prewarm warms, no-ops when resident, refuses at budget, evicts
nothing when refusing, ignores unknown labels) and +4 project UI (jump records
the hop, Esc crosses back, hop consumed). Confirmed the Esc-back checks fail with
the branch removed. 195/0 scenarios, 27/0 project UI, 36/0 index, 27/0 pool,
33/0 project.

Left open: project-level persistence across sessions.

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