The incident: mid-session on battery the pstate held turbo down (~2.5 GHz, 60°C,
CPU pressure ~0 — the set never needed more). Plugging in on AC at 4% silently
lifted that limit; the SAME workload jumped to 4.4–4.5 GHz and 85–93°C with fans
maxed. Nothing was working harder — the platform just reset the frequency cap
underneath our "cool" mode, and the tooling never noticed. set_mode() was
fire-and-forget: it wrote the cap once and moved on.

Fix: treat the perf mode as DESIRED STATE and reconcile it whenever the platform
can drift it (AC/battery flip, resume-from-suspend), with a periodic drift check
as the safety net.

perf.py
  - write_desired()/read_desired(): persist the user-chosen mode (~/.cache/
    parvagues/perf-desired). "normal"/absent == maintenance off, so we never
    fight a deliberate switch back to platform defaults.
  - set_mode() records intent on every successful switch.
  - on_ac(): mains state from /sys/class/power_supply/A*/online.
  - reconcile(force): reassert desired if detect_mode() drifted (idempotent —
    "cool holding" when already in force, so no thrash).
  - run_watcher(): daemon loop. Reasserts (force) on a power-source edge and on
    resume (detected via a monotonic-clock gap), settling 2s for the platform;
    periodic force=False drift check otherwise. Seeds desired from the live mode
    on startup so an already-cool session is maintained immediately.

bridge.py: launch run_watcher() as a daemon thread from `serve` (the always-on
--user service is the right home; the tray is just a face).

perf-tray.py: record intent (write_desired) on a successful tray switch too, so
both faces agree and the watcher respects tray-initiated changes.

Bonus: because "cool" also RT-schedules scsynth/sclang/Ardour/PipeWire, every
reassert re-guarantees audio-chain priority after a power event — cheap insurance
against post-transition xruns.

Validated: compiles; reconcile idempotent; force reassert exercises the real
perf-audio path (cool->cool, no disturbance); watcher thread confirmed live in
the daemon with "on_ac=True desired=cool". Real power-transition proof lands on
the next unplug/replug (logged to the bridge journal).