Preserve board state across canvas remounts

[ojowwalker77]

Summary

• retain one controller-owned workspace across every full ComposerCanvas remount
• synchronously flush live board text and geometry before replacing the SwiftUI root
• preserve the same BoardViewModel, per-board undo/redo cache, CanvasBridge target, and pan/zoom state across theme, language, system-appearance, and full-screen recovery remounts

Root cause

PanelController rebuilt the canvas with ComposerCanvas(), which created a new @StateObject BoardViewModel. A pending debounced save captured the discarded model weakly, while undo/redo and viewport state lived only on that model/view, so a remount inside the 400 ms debounce window could lose the latest edit and history.

Verification

• swift test --filter CanvasWorkspaceSessionTests — 2 passed; every remount trigger covers latest text/geometry, stable model identity, undo/redo, viewport, bridge routing, cancelled debounce, and protected unreadable payloads
• ./script/check — release build + 196 tests passed
• ./script/verify — signed release bundle validated
• ./script/build_and_run.sh --verify — staged app launched
• real app: Light→Dark→Light and System→English→System remounts kept board/settings context intact; original preferences restored
• Splus staged audit — 0 findings, 15/15 changed export contracts traced

Deliberate non-goals / risk

• This does not change board/dock window composition; [P0] Restore the sibling board and Agent/Settings dock panels #93 owns that restoration.
• Transient in-progress gestures remain view-local and reset on remount; committed pan/zoom and all board/history state survive.
• Rollback is the single commit.

Closes #96