feat: add revisioned Bridge protocol

[0ldtimer]

Summary

• add a versioned fm-bridge.v1 JSON boundary over the existing fleet snapshot
• project stable task identities, fixed-size revisions, evidence pointers, freshness, and legal capabilities
• add stale-safe, digest-bound, idempotent command handling
• keep sign-off separate from merge and leave guarded merge unsupported

Validation

• tests/fm-bridge.test.sh: passed
• tests/fm-fleet-snapshot-view.test.sh: passed
• bash -n bin/fm-bridge.sh bin/fm-bridge-lib.sh tests/fm-bridge.test.sh: passed
• bin/fm-lint.sh: passed with pinned ShellCheck 0.11.0

Safety

• requests are bounded to 64 KiB
• command IDs use a bounded safe identifier grammar
• per-command atomic locks close concurrent replay races
• identical retries replay the recorded outcome
• stale task or evidence revisions reject without mutation
• feedback routes through fm-send; defer and sign-off create durable review records

New concepts

Bridge protocol is an anti-corruption layer that gives voice clients a supported machine boundary without making terminal prose authoritative.

Command digest binding ensures a command ID can replay only the exact request it originally represented.