chore(architecture): tighten boundary guardrails

[limityan]

What changed

• keep the retired bitfun-core/tauri-support feature as a downstream-compatible no-op while removing the unused Tauri dependency
• make the remote-workspace LegacyUnaudited baseline an exact monotonic ratchet and remove 12 commands that already graduated to explicit policies
• expose explicit local commands for running the repository boundary check and its tests
• document why relay-service intentionally uses separate SQLx and rusqlite paths for trusted control-plane SQL and sandboxed Page Function SQL

Why

The architecture audit found an unused platform dependency in shared core and slack in the frozen remote-workspace policy baseline. The latter allowed an already-audited command to regress to LegacyUnaudited without failing the existing no-growth test. This change closes that gap without breaking the public Cargo feature name.

Verification

• node scripts/check-core-boundaries.mjs
• node --test scripts/check-core-boundaries.test.mjs (16 passed)
• cargo check -p bitfun-core -p bitfun-relay-service
• cargo test -p bitfun-desktop --lib remote_workspace_policy (6 passed)
• node scripts/check-repo-hygiene.mjs
• git diff --check