Multi-Main as a first-class finals format (A/B/C-mains by quali rank, bump-ups)

[ryan-johnson2]

Make MultiGP Multi-Main a first-class, next-iteration finals format. Multi-Main is distinct from elimination: a finals split by qualifying rank — A/B/C-mains racing in parallel for placement within their tier, with bump-ups between tiers. It needs its own design, not a reuse of the elimination chain.

Pairs with double-elim as the two next-iteration must-have formats with real model depth.

See the Decisions doc (decisions.html) on the devel branch for the full rationale and the format-set context. (decision D14).

[ryan-johnson2]

Update: a richer, bracketed take on multi-main shipped via composition rather than the flat-mains engine — each main is its own single-elim bracket wired bottom-up by new range/union seeding rules (#323), proven + live-verified at 64 pilots (#324). Per-tier configurability (tier count, per-tier win condition) + a builder wizard are tracked in #325. The flat single-heat multi_main engine (this issue's original plan) also exists as a simpler option.