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.

scripts/type-equiv.manifest.json

@@ -21,6 +21,7 @@
     { "doc": "docs/core-data-structures/llm-streaming.md", "symbol": "AppIdentity", "source": "packages/llm/llm/src/attribution.ts" },
 
     { "doc": "docs/core-data-structures/session.md", "symbol": "SessionEventMap", "source": "packages/core/session/src/types.ts" },
+    { "doc": "docs/core-data-structures/session.md", "symbol": "EpochHeader", "source": "packages/core/session/src/types.ts" },
     { "doc": "docs/core-data-structures/session.md", "symbol": "TodoItem", "source": "packages/core/session/src/types.ts" },
     { "doc": "docs/core-data-structures/session.md", "symbol": "SessionEvent", "source": "packages/core/session/src/types.ts" },
     { "doc": "docs/core-data-structures/session.md", "symbol": "TurnTriggerMap", "source": "packages/core/session/src/types.ts" },