[mottihoresh]

The audit of the last day's firmware and client PRs found the same gaps over and over. This PR writes the rules that prevent them into .claude/skills/firmware/SKILL.md, so every session that loads the skill gets them.

Rules added or changed

• Spec sync by behaviour, not only by value. Grep the old behaviour's wording as well ("coalesc", "broadcast", "once per boot", renamed listeners) across FirmwarePlan, TechSpec, docs/, READMEs and docblocks. Update every TechSpec that describes the behaviour. Amend or supersede every 00-decisions.md entry the change overrides. fix(firmware)!: sim output reaches CAN, inputs win the bus and are never lost silently (#389) #392 left D7/D24 describing removed coalescing, and fix(firmware): sync nodes one at a time so SYNC_REQ answers can't overflow the bridge (#387) #390 left D5 describing a broadcast SYNC_REQ.
• Pages that go stale fastest: compatibility.md, guides/bring-up.md, guides/new-control-type.md, build-flags.md and FirmwarePlan 09 join index.md, control-types.md and can-bus.md.
• Closing a ticket: tick every Done-when box with its evidence, or move the unmet item to a named follow-up, for every ticket a PR closes. Say when a check was substituted or not re-run.
• Release facts come from the tag: git merge-base --is-ancestor <commit> firmware-vX. firmware-v0.2.0 was cut 7 minutes before feat(firmware)!: v1.0 freeze fixes — firmware version on the bus, config structs, pinned dependencies (#295) #359. Releases before 1.0.0 are test-only.
• No new reporting without a need: no new frames, fields, flag bits or DIAG growth for visibility's sake. Loss counters stay on the board. "Never lost silently" means counted on the board.
• Responsive inputs first: input loss or latency fails; a late or once-dropped output that catches up is acceptable (D31).

Skill text only. check_doc_refs.py: OK.