docs(tech-debt): update technical debt analysis for v1.6.1

- Archive TECH-DEBT-v1.5.0.md as TECH-DEBT-v1.5.0-ARCHIVED.md
- Create TECH-DEBT-v1.6.1.md with comprehensive analysis
- Document completed items from v1.5.0 sprints (all critical/high resolved)
- Identify 25 remaining items (0 critical, 5 high, 8 medium, 12 low)
- Update code references from v1.5.0 to v1.6.1 in nat.rs and af_xdp.rs
- Estimated remediation reduced from ~330 to ~160 hours

Key improvements since v1.5.0:
- Transfer cancellation (TC-001) fully resolved
- Test count increased from 1,303 to 1,626 (+24.8%)
- React frontend tests added (62 tests)
- WRAITH-Chat and mobile clients identified as main debt areas

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: doublegate

crates/wraith-core/src/node/nat.rs

@@ -436,7 +436,7 @@ impl Node {
         //    - Validate candidate addresses against spoofing
         //
         // See: RFC 8445 (ICE), RFC 8838 (Trickle ICE)
-        // See: to-dos/technical-debt/TECH-DEBT-v1.5.0.md TM-001
+        // See: to-dos/technical-debt/TECH-DEBT-v1.6.1.md TM-001-DEFERRED
 
         Ok(remote_candidates)
     }

crates/wraith-transport/src/af_xdp.rs

@@ -550,7 +550,7 @@ impl AfXdpSocket {
         // - Current UDP transport provides functional baseline for all platforms
         //
         // See: https://docs.kernel.org/networking/af_xdp.html
-        // See: to-dos/technical-debt/TECH-DEBT-v1.5.0.md TH-006
+        // See: to-dos/technical-debt/TECH-DEBT-v1.6.1.md TH-006-DEFERRED
 
         Ok(Self {
             fd,