feat(example)+docs: pvd_tool on the modern stack; chapter 42 teaches the daemon path

C9 — the flagship vendor example now demonstrates the intended integration
shape. examples/pvd_tool/main.cpp: 1093 -> 570 lines. The 466-line
hand-registered handler section and the hand-wired Server/Router/emit
plumbing are gone, replaced by EquipmentRuntime + register_default_handlers
(the example now serves all 56 handlers, up from its hand-picked 51) +
commands.set_handler for the START-runs-the-recipe behaviour (was a
hard-coded S2F41 router override). All domain logic — sensor simulator,
recipe runner, alarm threshold monitor, EPT cycler, Prometheus gauges —
unchanged. pvd's SVIDs 1/2 and CEIDs 400/401 match the roles: defaults, so
the built-ins bind with no config change. Verified: builds clean, boots
("registered 56 handlers", config loaded, EPT cycling), HSMS :5000 accepts,
metrics :9090 answers HTTP 200. logfn flushes per line so docker/CI logs
are visible immediately.

Writing project — new tutorial chapter docs/42_vendor_daemon_and_clients.md:
why a daemon (the host-timer argument), the proto contract and the HCACK-4
command semantics, the Python client walkthrough, EquipmentRuntime +
capability registration + roles:, the threading contract (posting API /
read_sync / hooks-on-io-thread) and primary-vs-observer slots, and a
which-tier-do-I-pick table. Indexed in 00_index Part 4. Refreshed the three
spots that still described pvd_tool's old "51 handlers in ~460 lines" shape
(ch35, ch41, pvd README) — drift killed in the same commit that made it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/00_index.md

@@ -138,6 +138,7 @@ how to run it.
 |---|-------|--------|
 | [40](40_building_running_demo.md) | Building, running, the demo | Docker setup, the two-container demo, every transaction it walks through. |
 | [41](41_integration_hardware_mes_production.md) | Integration | Wiring sensors and recipes, talking to a real MES, HSMS-GS for multi-MES, persistence layout, monitoring, security hardening, performance envelope. |
+| [42](42_vendor_daemon_and_clients.md) | The vendor daemon + language clients | secs_gemd, the gRPC API and the HCACK-4 command contract, the Python client, EquipmentRuntime + per-capability registration, the threading contract, which tier to pick. |
 
 ### Part 5 — Reference