The fusion quorum-clamp/failure-detail root cause reported in #6454 was
already fixed and merged via #6521 (open-sse/services/fusion.ts already
carries Math.max(1, cfg.minPanel) + per-member failure reasons on this
branch). That merge never landed a CHANGELOG bullet for #6454 itself.
Backfills the missing bullet and adds a regression test at the exact
repro scale (11-member fusion-free-style panel, 2 cooling / 9 healthy)
to lock in that a cooling minority no longer sinks a healthy majority,
while a genuinely all-failed panel still returns the documented 503.
