fix(session): coalesce resize storms, gate size pushes to the owning pane

[yicheng47]

Fixes #373.

The v0.4.7 [launch-dims] instrumentation proved the launch-resume corruption was not a fork-width bug: forks landed at caller-supplied real widths, and what destroyed the panes was ~50 cols-gate ring purges in 90 seconds of post-launch layout churn — each purge dropping the resume transcript claude had just repainted. Purge semantics themselves stay (impl 0020 / #306): given genuinely stale-width bytes, purging remains correct. This PR removes the churn that fed it.

The three defects → three fixes

1. No coalescing → backend trailing debounce with round-trip skip. Cols changes for clears-on-resize runtimes park in SessionState::pending_resize; every further push folds in and extends the deadline; a per-storm settle thread applies one final ioctl after 175ms quiescence and purges only when the settled cols differ from the cols the ring holds. A drag that ends where it began costs zero purges and exactly one restoring repaint (rows-nudge, since a same-size TIOCSWINSZ is a kernel no-op and the owner pane cleared its viewport during the drag). Rows-only resizes stay synchronous — preserving the activation dance's SIGWINCH nudge — as do shells. update_last_size stays prompt, and a failed ioctl still cannot purge. The settle runs as one critical section under the session state lock, aborting on killed/resuming/handle-gone: kill sets killed under that lock before runtime.stop begins and resume sets resuming before runtime.spawn can overwrite the id→PTY mapping, so a stale settle can never physically resize a replacement child.
2. Transient garbage fit (78×23 against a ~209-col session) → placement settle-gate. createPaneGeometry (moved to src/lib/paneGeometry.ts) holds unplaced wrappers at 0×0 until they're positioned from a real pane-body rect — an unplaced absolute wrapper otherwise shrink-wraps xterm's default 80×24 canvas into a measurable, meaningless box. Zero size keeps the pane unmeasurable, so the existing rect guards hold every fit and push.
3. Two writers oscillating (76↔209↔76↔210) → single-writer per session. sizePushVerdict gates every backend size push: only the visible owning pane writes; hidden-pool wrappers and background tabs refit locally without writing; activation hands ownership over. Single-window case only — bug: two windows on the same chat — terminal size state desyncs across primary flips (stale lastPushed dedupe, dual-primary push race, wrong-cols replay) #313's multi-window protocol is out of scope.

Instrumentation extends #372 in the same runner.log: cols-gate purge … (N coalesced), cols-gate settle … round-trip, purge skipped, repaint nudged, and frontend [resize-gate] suppression/placement lines. Docs: correction note on impl 0024 decision 5, full decision record in docs/impls/0039-resize-storm-coalescing.md.

Tests

• Rust: storm collapses to one settled ioctl/purge; round-trip storm → zero purges + nudge pair; failed settled ioctl purges nothing and leaves the gate unmoved; rows-only stays synchronous and keeps the ring; settle thread fires unaided; settle inside kill's blocking-stop window aborts untouched; stale settle after kill+respawn touches neither the fresh PTY nor its ring.
• Frontend: sizePushVerdict (owner pushes, transitional/non-owner push nothing), shouldClearViewportBeforePush (clears pair only with real pushes), createPaneGeometry placement gate.

Checks

cargo test --workspace 562 passed / 0 failed · pnpm exec tsc --noEmit clean · pnpm run lint 0 problems · pnpm test 258 passed. Peer-reviewed via Runner crew: two must-fix rounds (round-trip blank viewport, settle/lifecycle race) resolved; final review clean.

Manual smoke (post-merge)

• Drag sidebar / split gutter / window edge across a live claude-code pane: scrollback survives, at most one repaint at drag end; a round-trip drag keeps its full ring.
• Quit + relaunch with auto-resume: log shows pane-placed before first-fit, coalesced settle lines, no transient-width purges; scroll history intact.

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