notion-react: deep-nesting (3+ levels) e2e coverage + cache schema-mismatch e2e

From principal-engineer e2e review (packages/@overeng/notion-react/context/testing/e2e-review.md, commit a076493e). Two critical e2e coverage gaps that could mask regressions:

Deep nesting (3+ levels)

The sync diff recurses through diffChildren, but no e2e exercises nesting past 2 levels. The closest existing test (columns) is 1 level. Real Notion pages commonly have column_list > column > toggle > callout > list > paragraph chains; a subtle reorder/update bug at depth 3+ would only surface in production.

Add:

• deep-nesting/cold 3-level — full round-trip, assert shape + children counts at every depth.
• deep-nesting/update-at-depth-3 — mutate leaf paragraph text, assert {updates:1, inserts:0, appends:0, removes:0}.
• Optional 5-level stress variant to lock depth bounds.

Cache schema-version mismatch

sync() has a documented fallbackReason: 'schema-mismatch' path (src/renderer/sync.ts:136), invalidating and re-seeding when schemaVersion !== CACHE_SCHEMA_VERSION. It is only reachable by bumping the constant, and no test covers the fallback. If someone later touches the fallback logic, the bug is invisible until the next schema bump.

Add:

• cache/schema-mismatch — write a CacheTree blob with schemaVersion: 0 via FsCache, run sync(), assert fallbackReason === 'schema-mismatch' and the on-disk blob afterwards has schemaVersion === CACHE_SCHEMA_VERSION.

Scope

Page-renderer only; database sync is out of scope (use @overeng/notion-effect-client directly).

Start with packages/@overeng/notion-react/context/testing/e2e-review.md and the schema-mismatch path in src/renderer/sync.ts:136; inspect the existing page-renderer e2e coverage and FsCache usage. Add deep-nesting round-trip and depth-3 update cases plus the cache/schema-mismatch case, and verify the requested child counts, sync diff, fallbackReason, and reseeded schema version.