observerd: /observer/inbox + multi-coord stress phase 1c (priority-ordered events)

Phase 3 ask: real-world inbox-style event injection during the stress
test. Coordinators in production receive emails + SMS that trigger
contract responses; the substrate has to RECORD these signals AND
react with a search using the embedded demand. This commit lands the
endpoint and exercises it end-to-end in the stress harness.

observerd surface:
- New POST /observer/inbox route — accepts {type, sender, subject,
  body, priority, tag} and records as ObservedOp with
  Source=SourceInbox. Type must be email|sms; body required;
  priority defaults to medium. The handler ONLY records — downstream
  triggers (search, ingest, etc.) are the caller's concern, recorded
  separately. Keeps the witness role pure.
- New observer.SourceInbox = "inbox" alongside SourceMCP /
  SourceScenario / SourceWorkflow.
- Three contract tests on the new route (happy path / bad type / empty
  body), router-mount test extended, all green.

Stress harness phase 1c (Hour 9):
- 6 inbox events fire in priority order (urgent → high → medium):
    2 urgent emails (forklift Cleveland, production Indianapolis)
    1 high email (crane Chicago)
    1 high sms (bilingual safety Indianapolis)
    1 medium sms (drone Chicago)
    1 medium email (warehouse Milwaukee FYI)
- Each event:
    1. POSTs to /v1/observer/inbox (recorded by observerd)
    2. Triggers matrix.search using a parsed demand (the demand
       extraction is hard-coded for now; production needs a small
       LLM to parse from body)
    3. Captures both as events in the run JSON

Run #006 result (with v2-moe embedder + all phases including inbox):

  Diversity:
    Same-role-across-contracts Jaccard = 0.000 (n=9)
    Different-roles-same-contract Jaccard = 0.046 (n=18)
  Determinism: 1.000
  Verbatim handover: 4/4 (100%)
  Paraphrase handover: 4/4 (100%)
  Inbox burst:
    6/6 events accepted by observerd (200 status, all recorded)
    6/6 triggered searches produced distinct top-1 worker IDs
    distance distribution: 0.24 (Indy production) → 0.71 (Chicago
    drone surveyor — honest stretch since drones aren't in the
    5K-worker corpus, system surfaces closest neighbor at high
    distance rather than fabricating)

The drone-Chicago case is the architectural-honesty signal: when
the demand asks for a specialist NOT in the roster, the system
returns the closest semantic neighbor with a distance that flags
"this is a stretch." Coordinators reading distances see "we don't
have a great match here" rather than a confident wrong answer.

Total events captured: 67 (was 61 pre-inbox).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/observer/types.go

@@ -41,6 +41,12 @@ const (
 	// the workflow handler was casting a string literal to Source,
 	// which worked coincidentally but left the taxonomy implicit.
 	SourceWorkflow Source = "workflow"
+	// SourceInbox tags ObservedOps emitted by /observer/inbox — incoming
+	// real-world signals (email, SMS) that a coordinator would receive
+	// and act on. The handler only RECORDS the message; downstream
+	// triggers (e.g. matrix.search on the parsed demand) are the
+	// caller's concern, recorded separately.
+	SourceInbox Source = "inbox"
 )
 
 // ObservedOp is one entry in the observer's ring buffer (and JSONL