docs(proposals): note assistant-first provider constraint (#347 rev 1.1 validation)

Worldtree accepted the v1 wire validation green; contract rev 1.1 folds all three
consumer flags. Flag #1 surfaced a real provider constraint: first-message makes
the assistant seq 0 -> vLLM/openai_compat tolerate it (sindra unaffected), but
Anthropic-family providers 400 on an assistant-first array. Consumer must gate
first-message on provider compat; provider-agnostic normalization deferred.

docs/proposals/authored-message-injection.md

@@ -169,6 +169,15 @@ consumer side MUST tolerate per-tenant absence:
   undiscoverable in `/capabilities` for ungranted tenants (same hide-existence
   posture as the R27-V1A cross-owner pattern).
 
+**Provider constraint (first-message specifically).** A create-time first-message
+makes the assistant turn `seq 0`. Assistant-first-tolerant providers (vLLM /
+`openai_compat` — what our Tier-3 characters, incl. sindra, run) accept it out of
+the box. **Anthropic-family providers reject an assistant-first array** ("first
+message must use the user role") → the next generation `400`s. So the consumer
+must **gate first-message on provider compatibility** (or treat it as
+vLLM/`openai_compat`-only for v1). Sindra = `openai_compat` → unaffected;
+provider-agnostic normalization is a deferred engine follow-up.
+
 ---
 
 *This brief was cold-read-pressure-tested by a cross-frontier panel (Grok /