Port the enemy footprint decision-boundary probe to Rust (#227)

[wormeyman]

Port-first entry 2. test/enemyBaseField.spec.ts carries two claims, not one,
and only the first has a Rust counterpart. worstAbs/worstRel grade the
field's values, and fixtures.rs already reproduces those at both seeds in
three magnitude buckets. The second claim is footprintDisagreements: port and
game never fall on opposite sides of a cut at 0.05 through
min(v, ENEMY_PLACEMENT_CAP). Nothing in Rust graded that.

The gap is real rather than theoretical, because the two checks cannot see each
other. An aggregate tolerance passes a residual that still moves a position
across the cut, and a cut is what the overlay does with this field. The nearest
Rust check, the_enemy_fixture_is_mostly_basement_..., compares positive
counts, so two positions could swap sides and the count would not move.

Planted rather than predicted:

plant	result
bias the port by 0.011	a value sits 8.04e-4 from the cut
bias the port by 0.03	port and game fall on opposite sides - 3 at seed 123456

The second is the one that matters: 0.03 is 33x below the deleted spec's own
ABS_TOL of 1.0, so the aggregate arms would have passed it.

Two guards beside the probe, because a zero on its own can be luck. The count
of positions inside the cut is frozen per seed - 39 of 1032 at seed 123456, 37
at seed 777771 - so a port returning a constant cannot satisfy it. And the
nearest value to the cut is required to stay above 1e-3, so the f32 read cannot
be what decides a side.

This unblocks deleting test/enemyBaseField.spec.ts, which imports
src/noise/enemies/enemyBaseField.ts and therefore cannot outlive the #227
deletion. The spec stays until then; the coverage is now in both places.

Claude-Session: https://claude.ai/code/session_01UVcbv1pAhPUoCC6aBwZUtg

Co-authored-by: Claude Opus 5 (1M context) noreply@anthropic.com