ADR-0004: ratify AM-4a and AM-4b (maintainer decision)

Discharges the open item T07 raised. Values unchanged at 250,000 and
350,000; what was missing was a reviewed decision behind them, since
they had been set by the implementer in the commit that measured them
and that also changed the feature gating being measured.

The ADR supplies the argument T07's test requires -- why the targets
bind on FUTURE work rather than merely passing present work:

  AM-4a leaves 3,750 lines of headroom (1.5%), about one small crate,
  so any new shipped-runtime dependency breaches it almost immediately.
  That is intended: the shipped runtime should be effectively frozen.

  AM-4b leaves 32,979 lines (10.4%), deliberately looser -- dev tooling
  should absorb one moderate dependency without a spec change, not two.

  Both are ceilings on a quantity that only grows by choice. Nothing
  drifts across them; only adding a dependency does.

Falsification condition stated: if a later pass raises AM-4a to
accommodate a dependency it wants, that is the failure the ceiling
exists to catch, and the answer is an ADR arguing for the dependency.

First ADR written under the correction/retarget test; sets the shape.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

specs/GameKernel.md

@@ -135,7 +135,10 @@ Command (actor-tagged intent)
 
 ## 4. Acceptance metrics
 
-**On AM-4's retarget (2026-07-31).** AM-4 originally read "≤20
+**On AM-4's retarget (2026-07-31).** *Ratified by
+[ADR-0004](../decisions/ADR-0004-am4-ratification.md) on 2026-07-31; the
+headroom argument for why these ceilings bind on future work lives there.*
+ AM-4 originally read "≤20
 transitive crates", set against boardgame.io's 120 npm packages. That
 target was retired for two measured reasons. First, it was unreachable
 without undoing this spec's own contracts: K5 (seeded ChaCha) and K7