Add entrant slot descriptors to knockout stages

[ylynfatt]

Summary

Phase ② of World Cup–style advancement (follows #48). A knockout stage can now declare where each round-1 slot's team comes from via config.entrants — an ordered descriptor list where consecutive pairs are the round-1 matchups, so the list is the pairing template (1A v 2B, 1C v 3rd-placed, …).

• EntrantSlot value object: validates/normalizes the two descriptor shapes (group+position, best_placed+rank) and renders labels — Winner Group A, Runner-up Group B, 3rd Group C, Best-placed Phase 3a: Stage + Group domain (schema, models, policies) #4.
• SingleEliminationGenerator entrant mode: with entrants configured, fixtures generate as an all-TBD bracket sized to the list (power-of-two enforced) instead of seeding from season teams. AdvanceBracketWinner needs zero changes — same round/bracket_position coordinates.
• Bracket view: unfilled round-1 slots show their descriptor label in italics instead of bare TBD.
• Stage edit page: entrant builder for bracket stages, fed by the previous grouped stage (new sourceStage prop) — Classic template (1A v 2B …, group-mates in opposite halves), Seeded template (winners → runners-up → best-placed with standard bracket seeding, works for any power-of-two shape incl. 12 groups + 8 thirds = 32), and per-slot select editing.
• Validation: entrants only stick on bracket formats (silent drop otherwise, same pattern as other config keys); power-of-two count and per-slot shape enforced with clear messages.

Phase ③ (next PR): SeedStageFromGroups — resolving these descriptors against the group-stage standings with an admin confirm step, plus draw-mode pairing.

Testing

• 8 new EntrantSlot unit tests (labels, normalization, malformed shapes)
• 3 new generator entrant-mode tests (TBD bracket shape, coordinates, power-of-two guard)
• 8 new feature tests (validation, format gating, generation, bracket placeholder labels, sourceStage prop)
• Full suite: 421 passed; existing World Cup knockout page verified rendering with no regressions

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