docs(arch): decompose open-questions.md into per-OQ files under questions/

The monolithic open-questions.md (1310 lines, 47 OQs) was large enough to be
unmanageable, with high size variance (OQ-42 at 220 lines next to OQ-06 at 8).
Decomposed into one file per OQ under docs/architecture/questions/ (NNN-slug.md,
mirroring the ADR convention), with open-questions.md retained as the index:
theme-grouped tables plus a cross-theme Deferred/Blocked section that surfaces
the 6 deferred OQs with their Blocked-on conditions inline (the safe-exit
visibility surface). Per-OQ content moved verbatim; all 62 inbound links stay
valid (none used anchors). README's curated OQ summary dropped (now redundant
with the index tables).

Also seeds tasks/architecture/ with this task plus two follow-ups found during
the decompose: OQ-09/10 missing structured Blocked-on fields, and the
tasks/architecture/ blocker-task half of the Safe Exit protocol being
unenforced.

docs/architecture/questions/014-batch-operation-semantics.md

@@ -0,0 +1,8 @@
+# OQ-14: Batch Operation Semantics
+
+- **Origin**: [call-protocol.md](crates/call/call-protocol.md)
+- **Status**: resolved
+- **Door type**: Two-way
+- **Priority**: low
+- **Resolution**: Batch is a client-side pattern — multiple `call.requested` events with correlated IDs, responses arrive independently. QUIC's stream multiplexing provides the concurrency and ordering guarantees that batch would need. Batch-specific event types (e.g., `batch.requested`, `batch.responded`) would add protocol complexity without clear benefit over sending multiple `call.requested` events. See ADR-012.
+- **Cross-references**: ADR-012