ADR-0001: SIP strategy — native Rust core behind Kamailio + rtpengine

Record the SIP edge decision and align the docs:
- docs/adr/0001-sip-strategy.md: layered strategy (own Rust parser, rent the
  interop tail via a Kamailio + rtpengine SBC, grow native core behind the shield);
  pjproject FFI explicitly rejected for breaking the memory-safety thesis at the
  most exposed seam.
- PORT_PLAN §1 + open decisions: SIP row updated to the decided strategy.
- ARCHITECTURE: "biggest technical risk" now points at ADR-0001.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C2bfD7MkqEdfnMXxXBu456

docs/ARCHITECTURE.md

@@ -49,6 +49,8 @@ microservices for stateful/business/billing logic.
 
 ## Biggest technical risk
 
-The **SIP stack**. No mature pure-Rust option exists → FFI pjproject or front the edge
-with a battle-tested SBC initially; treat a pure-Rust stack as a long-term goal, not a
-v1 dependency. Everything else builds on the existing Rust media ecosystem.
+The **SIP stack** — **decided in [ADR-0001](adr/0001-sip-strategy.md)**: own the Rust
+parser from day one (the security thesis depends on it), front the public edge with a
+proven **Kamailio + rtpengine** SBC to absorb the interop tail, and grow the native Rust
+transaction/dialog core behind that shield. No pjproject FFI. Everything else builds on
+the existing Rust media ecosystem.