session: cache the derived history — one projection per node, frozen and shared

deriveMessages() no longer re-derives and re-clones the whole surface
per call: each node is projected exactly once, when first seen (O(new
nodes) per access), through the now-public deriveEventMessage — the
per-node pure function external reconstructors and the dev invariant
fold over a log prefix, so no two paths can disagree about a request's
messages. A surface rewrite (replace, invalidate) rebuilds, signalled
by SurfaceManager.replaceGeneration — monotonic, bumped by every folded
replace and by invalidate(), never reset. Callers get a fresh array
snapshot per call over SHARED deep-frozen messages: isolation of the
append-only log moves from per-call cloning to unrepresentability
(mutation throws), which the two isolation tests now pin directly.

docs/cordis-catalog/events.md

@@ -209,7 +209,7 @@ A session was created in the store.
 'session/created'(session: Session): void
 ```
 
-Source: [`packages/core/session/src/index.ts:37`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:38`](../../packages/core/session/src/index.ts)
 
 ### `session/event` — emit
 
@@ -221,7 +221,7 @@ An event was appended to a session log (sync, fire-and-forget). This is the per-
 
 Types: [SessionEvent](../core-data-structures/core.md)
 
-Source: [`packages/core/session/src/index.ts:45`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:46`](../../packages/core/session/src/index.ts)
 
 ### `session/flush` — parallel
 
@@ -231,7 +231,7 @@ Awaited durability checkpoint. The agent loop awaits `ctx.parallel('session/flus
 'session/flush'(session: Session): Promise<void> | void
 ```
 
-Source: [`packages/core/session/src/index.ts:55`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:56`](../../packages/core/session/src/index.ts)
 
 ## `subagent/*`
 

docs/cordis-catalog/services.md

@@ -163,7 +163,7 @@ get(id: SessionId): Session | undefined
 list(): Session[]
 ```
 
-Source: [`packages/core/session/src/index.ts:328`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:345`](../../packages/core/session/src/index.ts)
 
 ## `ctx.subagents` — `SubagentService`