feat(compass-app): close-time cancel of a window's in-flight calls (SEA-2035 M3b)

[rigel-mintaka]

This PR is part of a stack containing 2 PRs:

1. main
2. "feat(compass-app): close-time cancel of a window's in-flight calls (SEA-2035 M3b)" (this PR)
3. feat(compass-app): multi-window gtk3 e2e smoke gate (SEA-2035 M4) #369

When a webview window closes, cancel every in-flight bridge call
registered to it so no call's pump goroutine or server-side subscription
leaks for the app's lifetime. Without this, closing a window left its two
app-lifetime streams live forever, and the user can open and close
arbitrarily many windows (A4, F-close).

The bridge service gains CancelWindow(windowDispatcher): under the same
lock discipline as compass_rpc_cancel it collects and drops every
inflightCall whose stored per-call window handle equals the closing
window, then cancels each outside the lock. The window handle is the
comparable windowDispatcher value M3 already captures on the
inflightCall, so matching is a plain interface compare and no compass.v1
knowledge enters the shell (thin-shell safe); the cancel drives the same
id-keyed teardown sink, so a canceled pump stops silently and the
server-side subscription terminates. A nil window matches nothing, so
windowless/fallback calls are never swept by a close.

The close handler is attached inside the newAppWindow factory
(gtk3-tagged main.go) via OnWindowEvent(WindowClosing, ...), so every
window — the New Window menu and the restore loop alike — gets the leak
gate and it cannot be forgotten at a call site. The factory's return
value (the M1 seam) is now consumed here, so its return type and the
unparam suppression are removed. bridge_service.go stays //go:build unix
and application-free; only the handler wiring imports the Wails events
package.

Tests (unix-tagged, no webview): one service holds three long-lived
calls — two on distinct windows and one windowless — kept in-flight by a
blocking stub; CancelWindow(closing) sweeps only that window's call while
the other window's and the windowless call stay live, and CancelWindow(nil)
sweeps nothing. Event-gated on channels, no sleeps, race-clean.

Spec-impact: implements docs/designs/product/compass-multi-window/design.md
§M3b (close-time cancel of a window's in-flight calls); no spec change.
Ledger-impact: none.

Co-authored-by: Matt Wilkinson matt@rigel.build

[linear-code]

SEA-2035

[github-actions]

Compass engineering docs preview: https://seal-2035-mw-m3b-close-cance.compass-eng-docs.pages.dev

Deployed from seal-2035-mw-m3b-close-cancel at 224e9b2.