fix: enforce must-use-larger-die at turn start instead of after execution

[nodots]

Problem

Production game 2d98cf14-e262-433c-baf5-95a2f878311d froze in stateKind: 'moving' on the final play. The robot had one checker on its 2-point, 14 off, rolled [3,5]. Either die bears off the last checker, so maxDiceUsable = 1 and both dice were playable at turn start.

PR #129 added the must-use-larger-die validation in Play.pureMove, which rejects the smaller die after execution — but Play.initialize still offered the smaller-die move as ready. GNU's bearoff hint carries no die value, the AI matcher took the first ready move (die 3, roll order), and MustUseLargerDieError fired on every retry. RobotActionProcessor swallows robot-turn errors, so the game was never persisted and stayed stuck. Humans hit the same path: planMoveExecution uses readyMoves[0].dieValue.

Fix

1. Play.initialize: when maxDiceUsable === 1 and ≥2 distinct die values were playable at start, convert smaller-die ready moves to completed no-moves — the option the rules forbid is never offered.
2. planMoveExecution: when no explicit die was requested and the larger die is mandatory, prefer the largest ready die — heals stale plays persisted before this fix.
3. pureMove validation kept as defensive backstop.

Tests

• New describe block reproducing the production position (clockwise, both roll orders, counterclockwise mirror): smaller die becomes a no-move, the move bears off with the 5, 15 checkers off, no throw.
• Existing "smaller die rejected" test updated: the attempt now plays the larger die instead of throwing.
• Full suite: 68 suites / 448 tests green.