fix review findings: skip collided SCHEMA_VERSION 3; reject marker-less surface events

P1: both merge parents shipped SCHEMA_VERSION=3 for different layouts (surface
columns vs seed_length), so an on-disk 3 was ambiguous and wrongly accepted.
Bump to 4 (merged layout) so the version check rejects both sibling v3s.

P2: a surface-eligible event with no surfaceOp lands in the log but vanishes
from deriveMessages() (surface is the sole derivation path). The typed append
overload enforces the marker only when the type arg is a literal; it collapses
to optional when widened to the union (a caller iterating raw events). Guard at
runtime in both append() and the seed constructor — no backward-compat for
surface-less logs. Shared seed fixtures carry surfaceOp explicitly and the
appendLog helper forwards it verbatim (no synthesized default). Exports
isSurfaceEligibleType. Regression tests for all three, each verified to fail
on the unfixed code.

Gates: typecheck, test (1115), snapshot (14), doc-sync, lint, build, hygiene green.

docs/cordis-catalog/events-and-services.md

@@ -412,7 +412,7 @@ get(id: SessionId): Session | undefined
 list(): Session[]
 ```
 
-Source: [`packages/core/session/src/index.ts:300`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:321`](../../packages/core/session/src/index.ts)
 
 ### `ctx.subagents` — `SubagentService`
 

docs/rfc/implemented/architecture/2026-06-18-session-surface.md

@@ -51,9 +51,11 @@ The `repair.ts` module synthesizes `tool/result` closers for orphaned tool calls
 
 The dev-mode invariants plugin validates: `sourceEventSeqs` references (non-empty, no duplicates, references earlier events, references known seqs) and `surfaceOp` (replace `start ≤ end`, both endpoints are on the tracked surface, the range is non-reversed in surface position, and `sourceEventSeqs` includes every node the range shadows).
 
+Because the surface is the SOLE derivation path, a surface-eligible event that carries no `surfaceOp` marker is invisible to `deriveMessages()` — it would land in the log yet silently drop from history on resume/fork. `append`'s typed overload makes the marker mandatory for `SurfaceEventType` events at compile time, but only when the type argument is a SPECIFIC literal; when it widens to the `SessionEventType` union (a caller iterating raw events, e.g. `for (const e of log) append(e.type, e.data)`) the conditional rest collapses to optional and the compiler stops enforcing it. The marker requirement is therefore ALSO checked at runtime in two places: `append` itself throws on a marker-less surface-eligible event (covering the union-widening loophole), and the `Session` seed constructor re-checks the same invariant (alongside its seq-contiguity and JSON-serializability checks) so a seed/load/fork — which arrives as raw `SessionEvent[]`, bypassing `append` — is REJECTED rather than constructing a session that resumes with missing history. (No backward-compat path for surface-less logs: per the pre-release stance there is no persisted user data to preserve, so such a log is rejected, not upgraded.)
+
 ## Consequences
 
-- **`packages/core/session`**: New `surface.ts` (`SurfaceManager`), new types (`SurfaceOp`, `SurfaceIntent`), new fields on `SessionEvent`, modified `append()` (third required `SurfaceIntent` param), refactored `deriveMessages()` (surface path + legacy fallback), surface-aware `repair.ts`.
+- **`packages/core/session`**: New `surface.ts` (`SurfaceManager`), new types (`SurfaceOp`, `SurfaceIntent`), new fields on `SessionEvent`, modified `append()` (third required `SurfaceIntent` param), refactored `deriveMessages()` (walks the surface as the sole derivation path), surface-aware `repair.ts`. The seed constructor rejects a surface-eligible seed event missing its `surfaceOp` marker (see § Invariants).
 - **`packages/core/agent-loop`**: All surface-capable appends pass surface opts. Chunk seqs are collected for `assistant/message` provenance; `tool/call` seqs are captured for `tool/result` provenance.
 - **`packages/session-persistence/session-persistence-sqlite`**: Two new nullable TEXT columns (`source_event_seqs`, `surface_op`) on the `events` table; `SCHEMA_VERSION` bumped (bump-and-reject, no migration).
 - **`packages/support/invariants`**: Surface-related validation rules.

docs/rfc/implemented/testing/2026-06-22-fork-child-replay-seed-boundary.md

@@ -27,7 +27,7 @@ Record where a session's **inherited** prefix ends, persist it, and have the rep
 - **JSONL**: a `seedLength` field on the header line (`toHeaderLine`/`fromHeaderLine`).
 - **SQLite**: a `seed_length` column on the `sessions` table.
 
-The SQLite change is a breaking table-layout change, so `SCHEMA_VERSION` bumps **2 → 3**. Per the repo's pre-release stance (§ "Pre-release stance" in AGENTS.md) the backend **rejects** a non-current `user_version` on open rather than migrating it — there is no persisted user data to preserve, so no migration code is written (the existing reject-not-migrate path at `openDatabase` already enforces this; v1 and now v2 are both rejected).
+The SQLite change is a breaking table-layout change, so `SCHEMA_VERSION` bumps. This branch added `seed_length` under version **3**; it later merged with the session-surface branch, which had independently shipped its OWN version-3 layout (the `source_event_seqs`/`surface_op` columns). Because an on-disk `3` is ambiguous between the two sibling layouts, the merged build is version **4** (every column), and an on-disk `3` is rejected like any other non-current version. Per the repo's pre-release stance (§ "Pre-release stance" in AGENTS.md) the backend **rejects** a non-current `user_version` on open rather than migrating it — there is no persisted user data to preserve, so no migration code is written (the existing reject-not-migrate path at `openDatabase` already enforces this; v1, v2, and the collided v3 are all rejected).
 
 ### 3. Replay derives a child script after the boundary