feat: sign last_session_close out of hours (fix RAI-693)

[hardyjosh]

The broker positions endpoint exposes current_price with no
per-symbol timestamp. The d841a31 broker-mark cutover stamped
Utc::now() at fetch time and called that publish_time, which only
bounds the server's own freshness — max-staleness in the strategy
therefore always sees a fresh stamp regardless of whether the
underlying market data is hours stale.

This week's verification (45h crontab sampler across all four market
phases) confirmed:

• During the extended session (04:00–20:00 ET on a trading day) the
  broker mark legitimately moves with pre-market and after-hours
  quotes. Signing now is correct.
• During a full close (overnight, weekend, holiday) it freezes hard
  (31h × 4 symbols × zero changes in the weekend sample). Signing
  now lies; the strategy accepts an old close as fresh.

The original "stopgap" (only re-stamp when current_price changes) was
ruled out because broker marks can freeze for 15–45 min during real
RTH on mid-volume names (SGOV/COIN/AMZN), which would false-positive
strategies with a tight max-staleness window.

This PR sources the truth from Alpaca's /v1/calendar instead:

• New market_hours module: MarketHoursCache holds the extended
  session windows around today; publish_time_for(now) returns now
  inside an active window, the most recent past session_close
  outside. anchor_session_to_utc converts the calendar's HHMM ET
  strings through chrono-tz so EST/EDT transitions are handled
  correctly.
• AlpacaClient::fetch_calendar(start, end) reads /v1/calendar and
  materialises each row into a SessionWindow via the new module.
• Primed at startup with ±7 days around today (cold-start during a
  holiday still resolves to the previous trading day's close), then
  refreshed hourly in the background. A failed fetch logs and keeps
  the previous cache — matching the quote-poll loop's behavior.
• build_response_from_quote consults the cache at sign time. The
  fetch_time on QuoteData is no longer used for the signed stamp.

Cache stays empty until the first refresh succeeds, in which case
publish_time_for falls back to now — preserving pre-fix
behaviour rather than blocking the server during a transient calendar
outage.

Tests:

• Unit tests cover every branch of publish_time_for (active session,
  out of session, holiday Monday with no entry falling through to the
  prior Friday close, weekday overnight, both session boundaries,
  empty cache) and anchor_session_to_utc (EDT, EST, malformed
  HHMM).
• Integration tests confirm /context/v1 signs last_session_close
  in the default out-of-session test app, and signs the request's
  wall-clock now when the app is configured with an active session
  window.

Closes RAI-693.

Co-Authored-By: Claude Opus 4.7 noreply@anthropic.com

[linear-code]

RAI-693

[coderabbitai]

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• Cargo.toml
• src/alpaca.rs
• src/lib.rs
• src/main.rs
• src/market_hours.rs
• tests/integration.rs

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[hardyjosh]

• feat: sign last_session_close out of hours (fix RAI-693) #12 👈 (View in Graphite)
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