pxx 2.3.7

Done-signal early-exit — the second half of "clean loop termination". 2.3.6's
predecessor R-017 salvaged an over-worked run's terminal code (report
COMPLETED, not BUDGET_EXCEEDED) but left the over-work itself: a local coder
keeps calling tools past a passing solution until its per-turn budget is spent.
This stops it at the first objectively-complete edit-state.

Added

• pxx loop done-signal early-exit (R-031): a per-round coder session whose
  on-disk diff already passes the objective gates (scope + diff-cap + lint +
  tests) stops mid-session and reports COMPLETED, instead of burning the rest
  of its budget. Implemented as an injected oracle (SessionContext.done_check,
  built by run_loop, consulted by the native backend after each edit turn) —
  no model-visible tool was added, so the tool surface is unchanged. The
  loop's own review gate still runs on the result. _edit_objectively_done is
  shared with the over-work salvage (_overwork_salvageable now delegates to it).
• done_signal setting (default on; PXX_DONE_SIGNAL, TOML strict
  boolean): turn the early-exit off for a suite slow enough that a mid-session
  test run costs more than the rounds it saves. Only ever fires inside run_loop
  with a configured test command; single-shot pxx run is byte-identical.