Close COMPLIANCE.md gap: spooling (E30 §6.22)

Implements the largest functional gap from the compliance audit. The
equipment now queues events the host can't immediately receive (either
because there's no SELECTED session or because the demo's force-spool
flag is on) and transmits the queue on host request.

What's new

  include/secsgem/gem/store/spool.hpp
    SpoolStore: a deque queue with a configurable per-stream whitelist
    (so only streams 5+6 spool by default), a max_size cap with FIFO
    eviction on overflow, and a `force_spool` test flag.  Enqueue
    returns one of Queued / Dropped_NotSpoolable / Dropped_Full so the
    caller can fall back to live delivery when appropriate.  Drain
    pops the entire queue in FIFO order.  Two new ack enums:
    ResetSpoolAck (S2F44 RSPACK) and SpoolRequestAck (S6F24 RSDA), plus
    SpoolRequestCode (S6F23 RSDC, Transmit/Purge).

  data/messages.yaml + auto-regenerated messages.hpp
    S2F43 W   <L,n <B stream>>            Reset Spooling
    S2F44     <L,2 <B RSPACK> <L,a ...>>  Reset Spooling Ack
    S6F23 W   <B RSDC>                    Request Spooled Data
    S6F24     <B RSDA>                    Request Spooled Data Ack

  data/equipment.yaml
    `spool:` section: max_size + spoolable_streams list.  Two new host
    commands SPOOL_ON / SPOOL_OFF that flip the force-spool flag (these
    stand in for "host link down" in the demo without dropping TCP).

  include/secsgem/gem/store/host_commands.hpp
    Spec/Result gain an optional<bool> force_spool field.  S2F41
    dispatch returns the result, the server applies it after S2F42 is
    queued.

  src/config/loader.cpp
    Reads `spool:` from equipment.yaml; reads `force_spool` from each
    host_commands entry; populates SpoolStore + CommandSpec.

  apps/secs_server.cpp
    New `deliver_or_spool(msg, what)` helper.  emit_event and
    emit_alarm_set funnel through it: if force_spool is on (or there's
    no active session), msg.stream is checked against the spoolable
    list and the message is enqueued; otherwise it's sent live.
    Two new handlers:
      S2F43  parses the stream list, updates SpoolStore, replies S2F44
      S6F23  RSDC=Transmit drains and re-sends each as a fresh primary
             (posted on the executor so the S6F24 ack flushes first);
             RSDC=Purge clears the queue and acks.
    The S2F41 handler now also propagates result.force_spool into the
    SpoolStore.

  apps/secs_client.cpp
    Demo extended with 4 new steps after the FAULT branch:
      SPOOL_ON  -> S2F42 Accept
      START     -> S2F42 Accept; CEID 300 emission spooled (no live S6F11)
      SPOOL_OFF -> S2F42 Accept; queue still has the message
      S6F23(Transmit) -> S6F24 Accept; spooled S6F11 arrives next
    Then the existing S7F19/S7F5/S10F1/S1F15/Separate flow continues.

  tests/test_data_model.cpp
    Four new TEST_CASEs for SpoolStore (whitelist, FIFO eviction at
    max_size, drain ordering, force flag).

  tests/test_loader.cpp
    Confirms equipment.yaml's `spool:` section populates the store and
    `force_spool: true/false` flows through to dispatch results.

  COMPLIANCE.md
    Spooling moves from ⬜ to 🟡.  Adds S2F43/F44 + S6F23/F24 as ✅ in
    the message coverage matrix; calls out what's still missing
    (S6F25/F26 notification, automatic activation on HSMS NOT-SELECTED,
    persistent on-disk spool).

Verified

  - Tests: 73 cases / 383 assertions pass (+4 spool cases).
  - Demo (docker compose up server client) walks the full happy path
    and the spool path, observed in the server log as:
        spool: force_spool=true (depth=0)
        spool: S6F11 CEID=300 queued (depth=1)
        spool: force_spool=false (depth=1)
        S6F23 transmit: draining 1 messages
    and on the host side as the queued S6F11 arriving in the correct
    order after S6F24.

Known limitations (logged in COMPLIANCE.md)

  - Spool activation is manual via SPOOL_ON/OFF rather than
    automatically triggered by HSMS NOT-SELECTED.
  - No S6F25/F26 spooled-data-ready notification on re-SELECT.
  - In-memory only; an equipment restart loses queued events.

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

data/equipment.yaml

@@ -39,12 +39,25 @@ recipes:
 
 # Dispatched by S2F41. `ack` is the HCACK enum value. Optional `emit_ceid`
 # fires a CEID after dispatch (e.g. ProcessStarted on START), and optional
-# `set_alarm` activates an alarm (e.g. FAULT -> alarm 1).
+# `set_alarm` activates an alarm (e.g. FAULT -> alarm 1).  `force_spool`
+# (true/false) flips the spool store's test-force flag — for the demo this
+# stands in for "the host link has gone down" without actually dropping
+# TCP.
 host_commands:
-  - {name: START, ack: Accept,      emit_ceid: 300}
-  - {name: STOP,  ack: Accept}
-  - {name: PAUSE, ack: CannotDoNow}
-  - {name: FAULT, ack: Accept,      set_alarm: 1}
+  - {name: START,         ack: Accept, emit_ceid: 300}
+  - {name: STOP,          ack: Accept}
+  - {name: PAUSE,         ack: CannotDoNow}
+  - {name: FAULT,         ack: Accept, set_alarm: 1}
+  - {name: SPOOL_ON,      ack: Accept, force_spool: true}
+  - {name: SPOOL_OFF,     ack: Accept, force_spool: false}
+
+# Spool (E30 §6.22).  Streams listed here may be queued by the equipment
+# when the host is unreachable; queued messages drain when the host sends
+# S6F23 with RSDC=0.  max_size caps the in-memory queue (FIFO eviction on
+# overflow).
+spool:
+  max_size: 100
+  spoolable_streams: [5, 6]
 
 # CEID emitted automatically whenever the control state machine transitions
 # (i.e. on every change-handler call). Set to null to disable.