fix: make generate_mobs hang-proof for any level data

[kennethsinder]

What

The mob-placement loop in World.generate_mobs rejection-sampled a non-spawn platform whose x differed from the longest platform's, looping until it found one:

which = self.rng.randint(1, len(self.platforms) - 1)
while self.platforms[which][0] == longest[0]:
    which = self.rng.randint(1, len(self.platforms) - 1)

If every non-spawn platform shares the longest's x (e.g. a hand-built level whose platforms all stack at one x), no roll can ever satisfy the exit condition and the build hangs forever — an unrecoverable freeze, not a catchable crash.

Until now this was only guarded at the editor boundary (EditorModel.playability refused to test-play such a level). That left World.from_level_data itself unsafe for every other caller (tests, future code, a hand-edited .dat). This fixes it structurally in the engine so the editor guard is no longer load-bearing for safety.

Changes

• generate_mobs is now provably terminating for any input. It pre-computes whether any eligible (differently-placed) non-spawn platform exists and places no ground mobs when none do, instead of spinning. When eligible platforms exist it consumes RNG identically to before — so procedurally-generated levels stay bit-for-bit reproducible (the determinism tests confirm this).
• from_level_data fails legibly. <2 platforms now raises a clear ValueError instead of an opaque IndexError deep inside gather_platform_info (which seeds its extremes from platforms[1]).
• Editor guard relaxed + made honest. With the hang impossible in the engine, EditorModel.playability drops the distinct-x rule and just enforces the real floor (≥2 platforms). Levels previously rejected on safety grounds are now allowed.

Tests

• test_degenerate_level_does_not_hang — builds the exact all-same-x level that used to hang, on a watchdog thread (so a regression fails loudly rather than freezing the whole suite, since pytest-timeout isn't a dep) and asserts only the patrol fly is placed.
• test_single_platform_level_raises — the ValueError floor.
• Editor playability tests updated to the relaxed contract.

98 passed locally; ruff / ruff format / mypy clean; headless --frames 120 smoke exit 0.

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