Layer 3 TAS Skeleton Bootstrap

Concern: G-A2600-001's acceptance criterion #2 ("TAS-driven 30-frame wall-hold: pacX delta within 1 pixel of expected") needs a tests/tas/atari2600/Pac_Man.tas.json file conforming to the Layer 3 schema documented in [internal specification]. Until the file exists, /prove's Layer 3 path falls back to the universal coupling smoke (drive joystick 30 frames each direction, assert ≥1 position delta) — enough for L3 but not L4.

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