session: the request header becomes logged state — request/header events + fold/diff/apply

Every conversation request's non-content half (system prompt, tool
schemas, call config — the EpochHeader) is now recorded in the session
log: a 'request/header' full snapshot (reason 'initial' | 'resume' |
'fallback') anchors the fold at conversation birth and process
boundaries, and 'request/header-delta' events (system line-trim,
name-keyed tools delta, whole config) encode mid-run changes. The pure
trio — foldRequestHeader / diffHeader / applyHeaderDelta — reconstructs
the header any request was built under from the log alone; the writer
contract round-trip-verifies every delta with a 'fallback' snapshot
when the encoding cannot express a change (pure tool reordering), so a
well-formed log always folds cleanly. Canonical absence: empty system
and empty tools normalize to absent fields, matching request builds.

Persistence and cordis catalogs regenerated; SessionEventMap paste and
EpochHeader added to the core-data-structures session page.

docs/cordis-catalog/services.md

@@ -124,7 +124,7 @@ stream(options: GenerateOptions): AsyncIterable<StreamChunk>
 
 Types: [GenerateOptions](../core-data-structures/core.md) · [StreamChunk](../core-data-structures/llm-streaming.md)
 
-Source: [`packages/llm/llm/src/index.ts:78`](../../packages/llm/llm/src/index.ts)
+Source: [`packages/llm/llm/src/index.ts:80`](../../packages/llm/llm/src/index.ts)
 
 ## `ctx.sessionPersistence` — `SessionPersistence` (abstract seam)
 
@@ -163,7 +163,7 @@ get(id: SessionId): Session | undefined
 list(): Session[]
 ```
 
-Source: [`packages/core/session/src/index.ts:327`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:328`](../../packages/core/session/src/index.ts)
 
 ## `ctx.subagents` — `SubagentService`