feat(session): opt-in packed chunk rows in the JSONL log

Providers stream token-sized deltas, so a session log stores hundreds of
near-identical assistant/chunk lines whose JSON envelopes dwarf their
payloads (~56x measured on a real DeepSeek session, 73% of file bytes).

Add a lossless storage codec to dsh-session: packChunkRuns() folds each
run of >=3 consecutive same-block delta chunks into one storage row --
text-chunks / reasoning-chunks / tool-call-chunks, bare slash-less tags
like the header line's 'session' so rows cannot be confused with session
events -- and decodeStorageRecord() expands rows back to the exact
original events (seq0/time0 + dt gap array reconstruct every member's
seq/time; tool-call rows carry the run-constant id/name). The encoder
whitelists exact shapes and stores anything unrecognized verbatim; the
decoder validates row-tagged values and fails loud on malformation.

The JSONL backend gains a packChunks config (default false). Writing
packs only when enabled -- default-off output stays byte-identical to
the previous layout, so snapshot goldens are untouched. Reading is
layout-blind: scanLog always decodes rows and now checks seq contiguity
with a cursor instead of the line index, so packed, unpacked, and mixed
files all load identically. Fixture readers (llm-replay parseSessionLog,
acp-snapshot normalizeSessionLog) share the codec; the normalizer zeroes
a row's time0/dt exactly like an event's time. The two demo bundles
plumb packChunks from cordis.yml to the backend.

Measured on a real coding session: 105 KB -> 42 KB (-60%), 475 lines ->
74, with reasoning/tool-call heavy sessions saving the most. Covered by
example + fast-check round-trip codec tests, backend packed/mixed/torn-
tail specs, and an end-to-end demo run loading a packed log through a
default-config backend.

docs/cordis-catalog/events.md

@@ -245,7 +245,7 @@ Creation announcement during session publication. A synchronous throw vetoes and
 'session/created'(this: Scoped<Session>, session: Session): void
 ```
 
-Source: [`packages/core/session/src/index.ts:47`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:49`](../../packages/core/session/src/index.ts)
 
 ### `session/disposed` — emit
 
@@ -255,7 +255,7 @@ Emitted once when an announced session leaves the store, including publication r
 'session/disposed'(this: Scoped<Session>, session: Session): void
 ```
 
-Source: [`packages/core/session/src/index.ts:57`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:59`](../../packages/core/session/src/index.ts)
 
 ### `session/event` — emit
 
@@ -267,7 +267,7 @@ Post-commit, fire-and-forget append feed. The listener snapshot resolves before
 
 Types: [SessionEvent](../core-data-structures/core.md)
 
-Source: [`packages/core/session/src/index.ts:69`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:71`](../../packages/core/session/src/index.ts)
 
 ### `session/flush` — parallel
 
@@ -277,7 +277,7 @@ Awaited parallel durability checkpoint: every listener runs and the caller await
 'session/flush'(this: Scoped<Session>, session: Session): Promise<void> | void
 ```
 
-Source: [`packages/core/session/src/index.ts:79`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:81`](../../packages/core/session/src/index.ts)
 
 ## `subagent/*`
 

docs/cordis-catalog/services.md

@@ -200,7 +200,7 @@ list(): Session[]
 fork(source: SessionForkSource, boundary?: number, childSessionId?: SessionId): Session
 ```
 
-Source: [`packages/core/session/src/index.ts:564`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:566`](../../packages/core/session/src/index.ts)
 
 ## `ctx.skills` — `SkillService`