docs: add Phase 0 (structural debts from design review); fix CompleteCommand contract comment

Phase 0 captures the 2026-06-10 review: multi-observer callbacks (done for
the critical three), CI for the interop/conformance harnesses (the unit
suite is partly self-referential; the external validators are the real
oracle), table-driven handler conformance + message-level golden frames,
register_default_handlers decomposition per GEM capability + YAML role
bindings for today's magic constants, the post+future mutable-read pattern,
service relocation + TSan run_async daemon test, identifier-safe name
validation. CompleteCommand's proto comment described the rejected blocking
model; it now states the settled HCACK-4 contract.

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

docs/DAEMON_ROADMAP.md

@@ -34,13 +34,11 @@ host and stays conformant while the tool software restarts/upgrades/crashes.
 
 ## Known issues (found in the 2026-06-10 audit; honest list)
 
-- 🚧 **`GetControlState` cross-thread read.** The gRPC handler reads the control
-  FSM's state enum while the io thread may transition it — a narrow data race
-  (single enum read; benign on every real ABI, but TSan-visible and sloppy).
-  Fix: an atomic state mirror in `EquipmentRuntime`. NOTE: cannot piggyback on
-  `ControlStateMachine::set_state_change_handler` — that is a single slot
-  already owned by `register_default_handlers`; either add multi-handler
-  support or update the mirror inside the runtime's own wiring.
+- ✅ ~~**`GetControlState` cross-thread read.**~~ Fixed 2026-06-10: the runtime
+  keeps an atomic control-state mirror updated via an `add_state_change_handler`
+  observer (`HandlerSlot` primary+observers pattern), so the mirror survives
+  `register_default_handlers` claiming the primary slot. `control_state()` is
+  now safe from any thread.
 - ⬜ **Alarms have no name key.** `equipment.yaml` alarms carry only numeric
   `id` + freetext `text` (matches SEMI: ALID/ALTX; there is no standard short
   name). The name-based `SetAlarm`/`ClearAlarm` RPCs need an optional local
@@ -83,6 +81,42 @@ Open sub-decisions to settle while implementing:
 
 ## Plan — ordered next steps
 
+### Phase 0 — structural debts (from the 2026-06-10 design review; pay before sprinting)
+The review's verdict: architecture and API bets are sound, but two structural
+debts tax every later phase, and the most valuable tests aren't automated.
+
+1. 🚧 **Multi-observer callbacks** (THE structural blocker — hit twice already).
+   `HandlerSlot` (primary slot keeps legacy set_ semantics; append-only add_
+   observers survive it) — done for `ControlStateMachine` + PJ/CJ stores, plus
+   runtime atomic control-state mirror (race retired) and `add_link_observer`
+   (WatchHealth foundation). ⬜ Remaining: roll the same 3-line pattern onto the
+   other single-slot classes (comm-state, EPT, exceptions, substrates, modules,
+   carriers, E84) as each phase needs them — mechanical now that the type exists.
+2. ⬜ **CI the interop + conformance harnesses** (`tools/run_interop.sh` + lane:
+   host_vs_cpp_server, daemon_interop, secs4j, tshark, secs_conformance). The
+   unit suite is partly self-referential (our parsers validate our builders);
+   the external validators are the real oracle and currently run only by hand.
+   Highest leverage-per-effort item in the repo.
+3. ✅ **Fix `CompleteCommand` proto comment** — it described the rejected
+   blocking model; now states the HCACK-4 contract.
+4. ⬜ **Table-driven handler conformance test** ((request, expected-reply-shape)
+   pairs through `router.dispatch` for broad coverage of the 56 handlers) +
+   message-level golden wire frames (codec KATs exist; message-level don't).
+5. ⬜ **Decompose `register_default_handlers` per GEM capability** (it is a
+   relocated main(), not a designed component) and replace magic constants
+   (SVIDs 1/2 `refresh()`, CEIDs 400/401) with YAML role bindings
+   (`control_state_svid:`, `cj_executing_ceid:` …). Gradual; aligns with the
+   capability structure GEM itself defines (S1F19) and enables vendor subsetting.
+6. ⬜ **Standardize the mutable-read pattern** for daemon RPCs: post-to-io +
+   future with deadline (always truthful; latency irrelevant at SECS rates).
+   First consumer: `GetVariables` (Phase A1) — set the precedent there.
+7. ⬜ Move `apps/equipment_service.hpp` into the library tree
+   (`include/secsgem/daemon/`) once Phase B grows it; add a TSan-built
+   `run_async` + concurrent-RPC daemon test (today's daemon tests only poll()).
+8. ⬜ Validate names are identifier-safe in `ConfigValidator` (the Python
+   client's kwargs API depends on it); generalize the format-compliance test
+   to iterate ALL configured variables (property-style, same cost).
+
 ### Phase A — finish the universal daemon surface (small, unblock vendors)
 1. ⬜ `GetVariables` — needs the reverse `Item → proto Value` conversion
    (read via post-to-io + future, or serve from a daemon-side cache of last