notion-react: latent diff bugs — sibling blockKey collision + same-key type-change

Found during principal-engineer e2e review (packages/@overeng/notion-react/context/testing/e2e-review.md, commit a076493e). Two reachable correctness holes in src/renderer/sync-diff.ts:

1. Sibling blockKey collision (C1)

diffChildren builds cacheByKey as a plain Map keyed on instanceKey(inst, i). If two siblings share the same blockKey, one silently overwrites the other. No error, no warning, and the resulting op plan is non-deterministic relative to candidate order.

Reference: src/renderer/sync-diff.ts:160.

Options:

• Reject at buildCandidateTree with a typed error.
• Document "last-wins" and add a dev-mode warning.

Either is defensible; today's behavior is neither.

2. Same-key type change (C2)

When a cached node and a candidate node share a key but differ in type (e.g. paragraph → heading_2), the diff currently emits:

{ kind: 'update', blockId: prior.blockId, type: cand.type, props: cand.props }

Notion's blocks.update does not support changing a block's type — the API will 400. The diff should detect prior.type !== cand.type and emit a remove + insert pair instead of an update.

Reference: src/renderer/sync-diff.ts:193-203.

Repro scenarios

No e2e tests exist for either case. See e2e-review.md §"Concrete test scenarios" for suggested fixtures (diff/key-collision, diff/type-change-same-key).

Scope

Page-renderer only; this issue does not affect database sync (out of scope for this library — use @overeng/notion-effect-client).

Start with src/renderer/sync-diff.ts, especially diffChildren around line 160 and same-key handling around lines 193-203. Read packages/@overeng/notion-react/context/testing/e2e-review.md and its Concrete test scenarios, then add coverage for diff/key-collision and diff/type-change-same-key. Done means sibling key collisions have an explicit supported behavior and same-key type changes produce remove plus insert rather than an invalid update.