Design ruled by Andrey 07/17/2026 (lives in imagej-elphel: the capability
follows the hardware the project runs on - 224/137; other programs use the
same localhost socket/CLI):
- perfd/elphel_perfd.py: root systemd service; line protocol on
  127.0.0.1:48890 (PING/STATUS/HOLD measure|max/RELEASE); a hold is TIED TO
  THE CONNECTION (client crash = auto-release), refcounted, strongest
  profile wins. 'measure' = CPU performance (power-profiles-daemon hold,
  same call as the old cpu_perf_hold.py it supersedes) + GPU persistence +
  clocks LOCKED (default 2550 MHz; 5060 Ti burst boost measured ~2595 @
  23 W/49 C - the lock is about run-to-run +-1% reproducibility for
  per-item margin evaluation, not speed). 'max' = performance, unlocked.
  ExecStopPost=nvidia-smi -rgc so a dead daemon never leaves clocks locked.
- perfd/install.sh (one-time sudo, idempotent) + perfctl (unprivileged CLI:
  'perfctl hold measure -- <cmd>') + README.
- Java: com.elphel.imagej.common.PerfDaemon (plain TCP, Java-8, zero deps;
  Hold implements AutoCloseable); Eyesis_Correction startup probe (one
  console line - uses the daemon when installed, prints the sudo install
  command when not); CuasPoseRT preloaded RT harness holds 'measure' for
  the timed run, releases after the RT summary.
Tested end-to-end user-mode (daemon on a test port): protocol, Java client,
CPU hold engaged, GPU lock correctly permission-refused as non-root,
auto-release on disconnect. GPU lock path activates once installed under
root. mvn package + test PASS.
Co-authored-by: Claude Opus 4.8 <claude-opus-4-8@anthropic.com>
Co-authored-by: Claude Fable 5 <claude-fable-5@anthropic.com>