docs(lite): archive superseded lite design/planning docs out of git tracking

Consolidate the lite-wallet documentation down to the single active plan
(lite-wallet-implementation-plan-v2-2026-06-04.md). The 8 prior design/planning
docs — the superseded v1 plan, its runtime-promotion-matrix, the two phase2
runtime-bridge plans, and the four backend artifact/signing design docs — are
moved to docs/_archive/ (added to .gitignore), preserving them locally as
reference while decluttering the tracked tree.

The v2 plan's References section is rewritten to be self-contained: it points to
docs/_archive/ for the historical design docs and to the actual shipping
mechanisms (scripts/build-lite-backend-artifact.sh, lite_backend_artifact_*,
lite_bridge_runtime.cpp) so there are no dangling tracked links. No code,
CMake, or scripts referenced these docs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -40,4 +40,6 @@ asmap.dat
 /ObsidianDragon-agent/
 
 # macOS
-.DS_Store
+.DS_Store
+# Local-only archive of superseded lite-wallet design/planning docs (untracked)
+docs/_archive/