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/core/agent-loop/README.md

@@ -56,9 +56,11 @@ forever:
       drain steering
       assembly = systemPrompt.assemble({agent})  ⟵ renderPrompt(assembly) IS the full prompt
       await serial agent/pre-step        ⟵ surface mutation (compaction) outside the step
-      session('step/start')
-      request = waterfall agent/request
-      stream llm.stream(request) → session('assistant/chunk')
+      boundary = session.deriveMessages()   ⟵ reconstruction boundary: same sync frame,
+      session('step/start')                     strictly before step/start
+      config = waterfall agent/request       ⟵ frozen seed; return a replacement to switch
+      session('request/header'[-delta])      ⟵ the header event this request owes the log
+      stream llm.stream(freeze({header..., messages: boundary})) → session('assistant/chunk')
       message = waterfall agent/step-result
       session('assistant/message')
       each tool-call: session('tool/call')