feat(tournament): double-elimination bracket with Winners/Losers/Grand Finals

[Samaro1]

Summary

Implements full Double-Elimination bracket support in backend/modules/tournament/src/bracket.rs, including Winners Bracket, Losers Bracket, and Grand Finals with bracket reset.

What was implemented

• Bracket generation for 4–128 players producing valid match graphs for both odd and even participant counts.
• Seeded auto-byes: when the participant count is not a power of two (e.g. 12 players → 16-slot bracket), byes are distributed to the highest-seeded players in Round 1 rather than randomly.
• Winners Bracket routing: winners advance to the next Winners Bracket round; losers are routed into the Losers Bracket in seeded order without duplicate match pairings.
• Losers Bracket progression: losers alternate through the Losers Bracket across the correct rounds (WB losers drop to the appropriate LB round and pair against LB winners).
• Grand Finals with bracket reset: if the Losers Bracket champion defeats the Winners Bracket champion in the first Grand Final, a second Grand Final (bracket reset) is triggered; otherwise the tournament completes immediately.
• Fixed Losers Bracket sizing and winner routing for non-power-of-two sizes so brackets always resolve to completion (previously a 32-player bracket overflowed and 12-player brackets stalled).

Tests

Added a unit test suite validating complete tournament progression and bracket generation for 8, 12, 16, and 32 player double-elimination brackets, including:

• Valid Winners/Losers/Grand Finals match counts
• Auto-bye distribution to top seeds
• Loser routing into the Losers Bracket without duplicate pairings
• Full tournament progression to a winner (8, 12, 16, 32 players)
• Bracket reset triggering and completion when the Losers Bracket champion wins the first Grand Final

All existing single-elimination, round-robin, and new double-elimination tests pass (38 passed in the tournament crate).

Closes #1019

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