fix(reseed): force a bounded parse so an incomplete frontier still clamps stale folds

[mtskf]

What

Follow-up to #293. reconcileReseedFolds runs exactly once, synchronously, from applyDocument right after an external reseed. When the post-reseed parse frontier was incomplete (!syntaxTreeAvailable(state, doc.length) — a large/just-opened doc reseeded with a real diff), it returned [] and never re-attempted, so #293's stale-fold clamp silently reverted for that fold's lifetime: an externally-inserted sibling heading could stay hidden behind a stale over-wide fold until the user unfolded.

Fix

When the reseed changed the document and there are active folds, force one bounded complete parse at the call site before reconciling:

• foldedRanges(view.state).size > 0 gate — folding is opt-in, so the common no-fold reseed pays nothing (no forced parse).
• forceParsing(view, doc.length, RECONCILE_PARSE_BUDGET_MS = 500) — not a bare ensureSyntaxTree: ensureSyntaxTree advances the parse context but leaves syntaxTree(state) (which foldable() reads) stale; forceParsing republishes the completed tree. This mirrors the existing seam already documented in the repo (forceParsing … force AND publish) and the outline-panel.ts / list-transform.ts forced-parse precedents.

Design decision (force vs. a reactive retry ViewPlugin) recorded in .claude/docs/LEARNING.md. On a pathological multi-MB doc whose parse exceeds the budget, forceParsing returns false and reconcileReseedFolds's own syntaxTreeAvailable guard leaves the mapped fold untouched — the pre-existing accepted behaviour, recoverable by unfolding.

Tests

• New test/webview/cm/fold/reconcile-reseed-folds.test.ts: on a genuinely-incomplete parse (large doc, no pre-forcing) reconcileReseedFolds bails ([]); once the tree is complete it clamps the over-wide fold. Non-vacuity verified (flip → real FAIL).
• New (r11) in editor.test.ts: end-to-end through applyDocument on a large doc — canonical fold range taken from a separate oracle state so the live parse stays incomplete; asserts the frontier is incomplete pre-reseed, then that the inserted sibling is revealed post-reseed. Red-first confirmed. flushPending() + editPosts() guards against any edit echo.

Display-only, byte-identical round-trip. Full unit suite green (4147 tests); pnpm compile + lint clean.