feat(daemon): D10 carriers + E16 ops RPCs + stress test + virtual fab

Completes the daemon's GEM300 surface and adds two new test tiers.

D10 — E87 carriers: CarrierStore gains the HandlerSlot observer pattern
(add_id/slot_map/access_handler). The daemon's id-observer forwards host
S3F17 decisions onto the Subscribe stream as CarrierAction (PROCEED on a
Confirmed transition, CANCEL on CancelCarrier); ReportCarrier drives the
flow tool-side: WAITING creates the carrier + records the slot map,
IN_ACCESS/COMPLETE advance the access FSM (INVALID_OBJECT on unknown,
CANNOT_DO_NOW on an illegal transition).

E16 — operations RPCs: Describe (full name inventory: variables/events/
alarms/commands/constants + device header), FlushSpool (purge or drain),
SendTerminalMessage (S10F1 tool->host, honest CANNOT_DO_NOW when no host
and stream 10 isn't spoolable).

Stream responsiveness: Subscribe/WatchHealth poll at 100ms (was 500ms) so a
cancelled stream frees its sync-server worker thread promptly — this was
found by the new stress test, which hung under Subscribe churn at 500ms.

Tests:
- A randomized concurrent RPC stress case: 4 threads x 250 seeded ops
  (set/get/fire/alarm/control-state/describe + Subscribe churn), asserts no
  failed RPC and a still-responsive engine afterward; prints its seed; a
  strong TSan target.
- A virtual fab (interop/virtual_fab.py + the `fab` compose service /
  tools/spawn_fab.sh): N daemons, each with a secsgem-py host AND a
  secsgem_client tool, driven by seeded random traffic with end-to-end
  invariant checks (set/get round-trips, event->S6F11 and alarm->S5F1
  delivery, command->tool->completion). Verified green at N=3 (~150 ops/eq,
  all commands round-tripped, 0 violations). Wired into run_interop.sh
  (now 13 steps).

Also fixes the CI break from the previous commit: the Python-client lane's
test_values.py step lacked PYTHONPATH=clients/python (now step-level env).

Two bugs found and fixed while building this, both mine from this batch:
1. carrier test hung on a CancelCarrier of a still-NotConfirmed carrier — a
   self-transition the FSM doesn't signal, so the observer never fired and
   the stream Read blocked forever. Fixed to cancel a Confirmed carrier;
   the NotConfirmed edge is documented as a known E87 limitation.
2. the 500ms stream poll above.

Daemon suite 7 cases / 214 assertions; core 475 / 3097; virtual fab green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

proto/secsgem/v1/equipment.proto

@@ -83,13 +83,31 @@ service Equipment {
   // are long-lived objects you report against as the physical work proceeds.
 
   rpc ReportProcessJob(ProcessJobState) returns (Ack);   // E40 — job-based tools
-  rpc ReportCarrier(CarrierState) returns (Ack);         // E87 — carrier-based tools
+
+  // E87 — carrier-based tools. WAITING announces a physically-arrived
+  // carrier (creates it; idempotent — re-announce updates the slot map);
+  // IN_ACCESS / COMPLETE drive the access FSM. The host's S3F17 decisions
+  // (ProceedWithCarrier / CancelCarrier) come back on the Subscribe stream
+  // as CarrierAction.
+  rpc ReportCarrier(CarrierState) returns (Ack);
 
   // ---- Diagnostics --------------------------------------------------------
 
   // Live daemon/link status: distinguishes "host went offline" from "cable
   // unplugged" from "spool filling up". Streams a snapshot on every change.
   rpc WatchHealth(Empty) returns (stream Health);
+
+  // Everything this equipment is configured with, by name — for tooling,
+  // diagnostics, and client-side validation/autocomplete.
+  rpc Describe(Empty) returns (EquipmentDescription);
+
+  // Flush the spool: purge=true discards queued messages, purge=false drains
+  // them toward the host (requires a SELECTED session).
+  rpc FlushSpool(SpoolFlushRequest) returns (Ack);
+
+  // Equipment-initiated operator message to the host (S10F1). Fails with
+  // CANNOT_DO_NOW when no host is connected and stream 10 isn't spoolable.
+  rpc SendTerminalMessage(TerminalMessage) returns (Ack);
 }
 
 // ---- Values ----------------------------------------------------------------
@@ -260,6 +278,27 @@ message Health {
   }
 }
 
+// ---- Diagnostics & operations -----------------------------------------------
+
+message EquipmentDescription {
+  string model_name = 1;
+  string software_rev = 2;
+  repeated string variables = 3;   // SVID + DVID names
+  repeated string events = 4;      // collection-event names
+  repeated string alarms = 5;      // alarm names (or stringified ALIDs)
+  repeated string commands = 6;    // RCMDs the host may send
+  repeated string constants = 7;   // equipment-constant names
+}
+
+message SpoolFlushRequest {
+  bool purge = 1;                  // true = discard, false = drain to host
+}
+
+message TerminalMessage {
+  uint32 tid = 1;                  // terminal id (0 = main)
+  string text = 2;
+}
+
 // ---- Acknowledgement -------------------------------------------------------
 
 // Mirrors SEMI HCACK exactly. For non-command RPCs, only ACCEPT vs an error