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.

packages/support/acp-snapshot/tests/normalize.spec.ts

@@ -109,6 +109,27 @@ describe('normalizeSessionLog', () => {
     expect(out).toContain('"decision":"block"') // the decision is the behavior — kept
   })
 
+  it('zeroes a packed chunk row\'s time0 and dt gaps but keeps seq0 and payload', () => {
+    const row = JSON.stringify({
+      type: 'text-chunks', seq0: 7, time0: 999,
+      data: { turn: 1, step: 1, index: 0, dt: [212, 27, 0], texts: ['a', 'b', 'c', 'd'] },
+    })
+    const out = normalizeSessionLog(`${header({})}\n${row}\n`, ctx)
+    expect(out).toContain('"time0":0')
+    expect(out).toContain('"dt":[0,0,0]')
+    expect(out).toContain('"seq0":7') // seq0 is deterministic, like seq — NOT scrubbed
+    expect(out).toContain('"texts":["a","b","c","d"]')
+    expect(out).not.toContain('999')
+    expect(out).not.toContain('212')
+  })
+
+  it('zeroes time0 even when a malformed row carries no dt array', () => {
+    const row = JSON.stringify({ type: 'text-chunks', seq0: 1, time0: 999, data: 'not-an-object' })
+    const out = normalizeSessionLog(`${header({})}\n${row}\n`, ctx)
+    expect(out).toContain('"time0":0')
+    expect(out).not.toContain('999')
+  })
+
   it('leaves a non-hook event durationMs untouched (only hook/result is scrubbed)', () => {
     const ev = JSON.stringify({ type: 'tool/result', seq: 2, time: 5, data: { durationMs: 88 } })
     const out = normalizeSessionLog(`${header({})}\n${ev}\n`, ctx)