Southbound conformance: PAUSED code, eight-measure health, schema alias, renderable panels

[mbreissi]

Applies the amended SOUTHBOUND.md southbound conventions (the core adapter-core-enablers train) to the Modbus reference adapter. Per-item status:

Checklist status

1. PAUSED refusal code — MIGRATED. repoll on a paused instance now refuses with top-level code PAUSED instead of BAD_ARGS. This is a wire-visible breaking change: any client pinning BAD_ARGS for the paused-repoll case must move to PAUSED. Updated together: command_service.repoll(), tests/test_commands.py, docs/reference/messaging-interface.md (error-code table + repoll entry), AGENTS.md invariants, and the DESIGN.md D-M2/D-M3 registers (which previously pinned the old code as the breaking-change record).

2. Eight-measure southbound_health — MIGRATED. Was six measures; now exactly connectionState, publishLatencyMs, pollLatencyMs, readErrors, staleSignals, reconnects, writeErrors, signalsSubscribed.

• writeErrors (Count, 60, drained on emit like readErrors): DEVICE-PATH failures only. command_service._write_one now splits the encode step from the device write, so allow-list refusals, unresolvable refs, missing values, and caller-side encode errors never count — only a write that passed validation + the allow-list and was then rejected by the device (or lost to a dead link) does. Per-entry reply granularity and the WRITE_FAILED batch semantics are unchanged.
• signalsSubscribed (Count, 1, gauge): Modbus is a polling adapter, so this is the configured/polled inventory the session serves while connected, 0 while disconnected (ClientMetrics.set_signals_serving, fed from the instance config in device.py).
• Measure-pinning test (tests/test_health.py) and docs/reference/metrics.md updated in the same change.

3. #/$defs/instance schema alias — CONFORMED-ALREADY. config.schema.json already carries "instance": {"$ref": "#/$defs/device"} (landed on main in b9834ef, PR #5). No change.

4. Panel descriptor floor — MIGRATED. The trio existed but was below the renderable floor (bare signalGrid/treeBrowser, summary with a fields key, commandSummary with an actions key). Now: summary.rows (label/value), commandSummary.verbs, signalGrid with signalsVerb and subscriptionsVerb (both → sb/signals; the second is a descriptor-compat alias the shipped console reads — no sb/subscriptions wire verb exists) + readVerb → sb/read, treeBrowser with browseVerb/mode: "hierarchical"/rootRef: "root", and widget-level scope: "instance" on every command-backed widget. No widget advertises a writeVerb (none did before either). Kept the repo's existing view set/ids (overview/signals/diagnostics). Scope note: the diagnostics panel's pre-existing keyValueList widget is this repo's own extra view — kept (with an id/title), not expanded, since the floor doesn't cover it.

5. Hierarchical sb/browse — MIGRATED. The repo registered a treeBrowser panel but sb/browse served only pages. Added the hierarchical form per the python template reference, over the same configured inventory the paged mode serves: presence of ref selects it; ref: "root" returns the device node with contains refs to the (flat) signal inventory; a signal id is a known leaf (refs: []); an unknown ref is BAD_ARGS; depth clamps 1..4, maxRefs 1..1000; mixing ref/depth/maxRefs with cursor/max, or depth/maxRefs without ref, is BAD_ARGS. Paged behavior unchanged.

Validation

• Baseline (pre-change) on the worktree: 121 passed, coverage 95.47% — green.
• After: 130 passed, coverage 95.67% (gate 90%, pyproject.toml addopts; python -m pytest from the repo's own venv with the editable sibling edgecommons lib per CLAUDE.md).
• No gate lowered; 9 tests added (PAUSED refusal, hierarchical browse root/leaf/clamps/mixing, writeErrors device-path vs policy/encode semantics, panel floor assertions).

Follow-ups (out of scope per the migration spec)

• Migrating legacy .../control/* topics to the UNS inbox (standing Phase-5 item) — N/A here; this adapter already serves the UNS command inbox.
• Adopting registerScoped/setCommandAvailability (requires a core version bump).

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