feat(muya): progressively mount large documents (#4887) - #4946

Progressively mount documents with many top-level blocks in #4887, stacked on #4942. The complete logical state is available synchronously; DOM construction starts with a prefix and continues in scheduled chunks. This reduces initial DOM work without claiming to resolve every large-document freeze.

Updated through 37bd8816, on revised #4942 (21418d13). The shared state ownership and History changes live in #4942; the additional commits here implement state-based TOC, progressive mounting, and lifecycle/consumer fixes.

Behavior and contracts

• TOC, reference definitions, and Markdown/state serialization read complete JSON state. DOM readers see only mounted blocks. TOC entries carry a top-level index for on-demand mounting before scrolling, retaining develop's heading scroll-position calculation.
• Flush queued edits and re-read state before appending from the mounted child count. Pre-mount OT targets and their required successor from pre-dispatch state; suspend on-demand mounting during pick/drop to avoid applying operations twice.
• Deep-clone mounted slices because blocks retain mutable metadata. Budget counting uses an iterative traversal and stops once the prefix budget is reached.
• Replacement and destruction invalidate pending work. Synchronous flush/progress/completion callbacks can invalidate requests or finish the same document without duplicate continuation timers. Detached hosts remain supported.
• Cursor restoration stops before restoring old source/history if a callback invalidates its temporary document. Frontier navigation distinguishes a successor, real EOF (null), and cancellation (false): paragraph arrows, code Shift+Enter, and table Enter/ArrowDown abandon canceled requests rather than appending to a replacement/destroyed document. Tab and Delete safely stop on cancellation as well.
• Search, footnotes, and end-of-document consumers may finish mounting synchronously. Container-background clicks are inert while a tail is pending.
• muya-mount-progress / muya-mount-complete describe progressive mounts. Small/empty documents mount synchronously without these events; canceled mounts do not emit completion. Public ensureMountedThrough returns false when its request is invalidated.

Limits

The 4 ms check runs between top-level blocks; it is not a task-duration or interaction-latency guarantee. A giant list, table, paragraph, or code block remains atomic. Parsing, browser layout, and synchronous full-tree consumers can still block. Container-internal virtualization is outside this PR.

The original 1.8 s / 9.6 s figures are historical results from an older revision, not current measurements of first paint/input readiness. A new single-sample local comparison mounted 512 of 3000 independent paragraphs initially, while a 3000-item list and 1000-row table still mounted synchronously as single blocks. This illustrates scope, not a universal responsiveness guarantee.

Validation

At 1418941b, the full unit run passed 1934 tests; two existing tests exceeded 5 seconds during concurrent builds, then passed with unchanged timeouts in a single-worker rerun (14 tests across their files). All 1347 specification tests, product TypeScript, scoped lint, circular check, and Muya/desktop builds passed. Existing lint warnings remain.

26 Chrome tests passed, covering clipboard, export, search, 10k-paragraph completion, and pending-tail consumers. After the cursor fix, all three progressive-consumer browser tests and 16 Electron integration tests passed again (source mode, tabs/history, TOC, and anchor scrolling).

The final review reproduced a missing combination: frontier Enter + replacement/destruction during a mount callback. Three tests first failed, demonstrating unwanted paragraph insertion. 37bd8816 fixes this and adds 20 regression cases across seven exits, including same-document reentrant completion. On that final revision, 131 targeted tests passed across 10 files (mount/cursor/table navigation, true EOF behavior, and arrows); TypeScript and scoped ESLint also passed, with the existing arrow-handler complexity warning. The earlier full-suite/build/browser totals belong to 1418941b, not a new full run of the final small patch.