Unify session surface validation

packages/core/session/README.md

@@ -32,10 +32,10 @@ The store pairs announced creation with disposal, publishes each append, and pro
 
 Plain class (not a Cordis Service). Create via `ctx.sessions.create()`.
 
-- `session.append(type, data, opts?): SessionEvent` — synchronous, never blocks on I/O. At this durable boundary, data and surface metadata are lossless-JSON snapshotted and deep-frozen. For an attached session, a reentrant append during dispatch/observer publication rejects, and detach waits for that publication to unwind. Callbacks resolve before the log push; the push is the commit point, after which each observer failure is contained independently. Runtime surface validation covers widened unions and raw seed/load logs.
+- `session.append(type, data, opts?): SessionEvent` — synchronous, never blocks on I/O. At this durable boundary, data and surface metadata are lossless-JSON snapshotted and deep-frozen, then the same atomic surface transition used by replay validates marker shape, provenance, and complete replacement coverage before the log changes. For an attached session, a reentrant append during dispatch/observer publication rejects, and detach waits for that publication to unwind. Callbacks resolve before the log push; the push is the commit point, after which each observer failure is contained independently.
 - `session.deriveMessages(): Message[]` — the LLM message history, CACHED: each surface node is projected exactly once, when first seen (O(new nodes) per call; a surface rewrite rebuilds via `surface.replaceGeneration`). Returns a fresh array per call over shared, deep-frozen `Message` objects. Each projection reuses the already deep-frozen content in its durable log event, so no second deep clone is needed and a consumer still cannot mutate logged data. The surface is the single source of derived history — there is no raw-log fallback.
 - `session.deriveEventMessage(event): Message | null` — the per-event projection `deriveMessages()` folds: a fresh message wrapper that reuses the event's already frozen content, or `null` when the event produces none (a non-surface event, or an empty-content `assistant/message` hosting only usage). External reconstructors and the dev invariant fold the same function over a log prefix's surface, so no two paths can disagree about what a request's messages were (the reconstructability RFC).
-- `session.surface: SurfaceManager` — the derived surface, lazily rebuilt from `surfaceOp` markers in the log. Processes only new events (delta) on each access — the log is append-only, so prior events never change. `surface.replaceGeneration` is the rewrite signal: bumped by every folded `replace` and by `invalidate()`, never reset, so an incremental consumer comparing generations cannot be fooled.
+- `session.surface: SurfaceManager` — the derived surface, lazily rebuilt from `surfaceOp` markers in the log. It processes only new events (delta) on each access; event acceptance uses a separate manager with the same transition so validation does not eagerly mutate this public view. `surface.replaceGeneration` is the rewrite signal: bumped by every folded `replace` and by `invalidate()`, never reset, so an incremental consumer comparing generations cannot be fooled.
 - `session.events` — a cached, frozen array snapshot over deep-frozen events. Repeated reads without an append return the same array; an append invalidates the cache and the next read returns a new snapshot, while earlier snapshots stay unchanged. Neither a cast nor a retained reference can push into the live log or rewrite an accepted event.
 - `session.seq`, `session.id` — current sequence and readonly typed identity.
 - `session.header: SessionHeader` — detached, deep-frozen creation metadata (`version`, `id`, `createdAt`, optional `cwd`/`parentSession`/`seedLength`). Construction validates the durable record and requires its id to match `session.id`.
@@ -49,8 +49,7 @@ Durable values need one accepted representation, not a check followed by a secon
 - `SurfaceOp` — how a surface node entered the linked list: `'append'` (normal tail append) or `{ op: 'replace', start, end }` (replace nodes from `start` through `end` inclusive — both must be valid surface node seqs; `start === end` replaces a single node). Used by compaction to shadow old nodes without deleting them.
 - `SurfaceIntent` — `{ surfaceOp: SurfaceOp; sourceEventSeqs?: number[] }`, the required third parameter to `session.append()` for surface-eligible types.
 - `SurfaceNode` — `{ seq: number; prev: number | null; next: number | null }`, one node in the surface linked list.
-- `foldSurface(events)` — replay the canonical surface transitions into detached current nodes and actual replacement ranges, rejecting misplaced, missing, malformed, or positionally invalid `surfaceOp` metadata while leaving provenance validation to callers. `SurfaceManager` shares the same transitions while retaining its incremental cache.
-- `validateSurfaceMetadata(event, knownSeqs?, shadowedSeqs?)` — canonical structural and provenance check shared by session acceptance, surface folding, incremental invariants, and exact readers. It tags violations as `surface` or `provenance` so callers retain their error taxonomy; omit `knownSeqs` for local shape validation only.
+- `foldSurface(events)` — replay the one canonical surface contract into detached current nodes and actual replacement ranges. The same pass rejects misplaced or malformed metadata, empty or duplicate provenance, unknown or non-earlier sources, invalid positional ranges, and replacements that fail to cite every shadowed surface node; `SurfaceManager` shares the atomic transition while retaining its incremental cache.
 - `isSurfaceEvent(event)` / `isSurfaceEligibleType(type)` — the first narrows a `SessionEvent` to a fully-formed surface node (type is surface-eligible AND `surfaceOp` present); the second is the type-only check (is this one of the five `SurfaceEventType` values?), used to detect a surface-eligible event MISSING its marker — e.g. when validating a seed/load log.
 
 ### Request-header reconstruction (`request-header.ts`)