loop: every request is built from the log — boundary snapshot, header events, config-only waterfall

The loop is now transmission-stateless; a request is a pure function of
(session log, this step's rendered assembly, current AgentOptions):

- The reconstruction boundary is step/start: the messages snapshot is
  taken in the same synchronous frame immediately before the step/start
  append, so the request's messages are exactly the derivation over
  events[0..stepStartSeq) — an inject() from an agent/request listener
  (or any concurrent task) lands after the boundary and joins the NEXT
  request. This changes behavior for a synchronous step/start
  session/event listener that appends content (master derived after the
  append, so such a listener could reach the current request):
  agent/pre-step is the sanctioned seam for current-request content.
- agent/request is re-typed to config-only: (agent, turn, step,
  config: LlmCallConfig, next) → LlmCallConfig. The frozen seed comes
  from AgentOptions on a loop instance's first request (explicit options
  beat the logged baseline — fork overrides and resume reconfiguration
  stay correct) and from the log's folded header afterwards; listeners
  return a replacement to switch. Content shaping through the request is
  no longer expressible — model-visible content flows through the log
  channels.
- recordRequestHeader appends whatever header event the request owes the
  log before dispatch: an 'initial'/'resume' snapshot anchoring each
  loop instance, a round-trip-verified delta on change, a 'fallback'
  snapshot when the encoding cannot express it. Session.requestHeader()
  is the log's incrementally-folded baseline.
- Requests are deep-frozen before dispatch (deepFreeze exempts the
  AbortSignal — freezing one breaks AbortController.abort() outright);
  frozen + sessionId is the loop-built marker the dev invariant keys on.

Ported from #162 and re-anchored on the log: the append-extension /
frozen-end-to-end / compaction-resend / prompt-change property tests,
plus new specs for the boundary semantics, resume anchoring, and the
end-to-end theorem (every recorded request rebuilds byte-equal from the
log alone). Live cache-hit e2e (request-cache.e2e.ts) verified against
the real DeepSeek API. Snapshot goldens intentionally stale until the
single re-record after the compact/summary envelope lands.

packages/llm/llm/tests/call-config.spec.ts

@@ -33,6 +33,18 @@ describe('deepFreeze', () => {
     expect(() => { (value.a.b[1] as { c: string }).c = 'y' }).toThrow(TypeError)
   })
 
+  it('never freezes an AbortSignal: the live cancellation channel keeps working', () => {
+    const controller = new AbortController()
+    const request = deepFreeze({ model: 'm', signal: controller.signal })
+    expect(Object.isFrozen(request)).toBe(true)
+    expect(Object.isFrozen(controller.signal)).toBe(false)
+    let fired = false
+    controller.signal.addEventListener('abort', () => { fired = true }, { once: true })
+    controller.abort('stop')
+    expect(fired).toBe(true)
+    expect(controller.signal.aborted).toBe(true)
+  })
+
   it('passes primitives through and terminates on cycles', () => {
     expect(deepFreeze(42)).toBe(42)
     expect(deepFreeze(null)).toBeNull()