[baseball] make the AI's off-plate pitches actually miss the zone

[gazure]

Problem

the_ai_pitcher_throws_strikes_and_balls_in_a_sane_mix is flaky on main — it failed in roughly 1 run in 4 (measured 3/12, then 2/12 across 24 runs), always by overshooting the upper bound (76–79% vs. the asserted 75% ceiling).

It is not just unlucky sampling. ai_pitch_plan aims in the zone 60% of the time and "just off the plate" the rest, but the off-plate branch sampled a rectangle spanning x ±1.55 by y 1.05..4.05 — which fully contains the strike zone (x ±0.83, y 1.6..3.4). The zone is ~32% of that rectangle's area, so:

in-zone rate = 0.60 + 0.40 × 0.32 ≈ 0.73

The AI was throwing ~73% strikes when the mix intends 60%, landing right on the test's ceiling — hence a coin-flip failure rather than a clean pass or a clean fail.

Fix

Aim past one chosen edge (inside/outside/high/low) instead of sampling a rectangle around the plate, so a pitch meant to miss always misses. All four cases stay within AIM_LIMIT_*, so the_ai_pitcher_never_aims_somewhere_unreachable still holds. HBP_X is -1.75, beyond AIM_LIMIT_X of 1.7, so hit-by-pitch was never reachable by aim and is unaffected.

Also adds a_pitch_meant_to_miss_actually_misses, which pins the invariant directly rather than leaving a statistical band as the only thing standing between this bug and CI.

Verification

• Strike rate back to ~60%, comfortably inside the asserted 0.45..=0.75
• Baseball suite 30/30 runs green (was ~25% failure)
• cargo +nightly fmt --all -- --check clean; CI clippy reports no errors
• 99 baseball + 26 twotris tests passing

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