Ban-pick: weighted tuple pool policy per gamemode (Botlatro generation parity)

The ban-pick engine's default pool is a uniform random sample of every installed deck Back — including modded decks — with no exclusions, no weights, and no stake dimension. Botlatro (the Discord matchmaking bot) already runs real pool policy for its tuple-ban queues; this issue is about giving consumer mods the same power in-game, per mod and per gamemode.

## Reference: what Botlatro's generation enforces (`src/utils/TupleBans.ts`)

- Allowed stakes: a fixed subset (White/Green/Black/Purple/Gold — 5 of 8).
- Allowed decks: `decks` table minus per-queue `banned_decks` minus match-step bans.
- Weighted roulette-wheel selection via per-queue `deck_mults` / `stake_mults`.
- Occurrence caps in the generated list (9 tuples): max 4 of any stake (`< (tupleCount-1)/2`), max 3 of any deck (`< (tupleCount-3)/2`), no duplicate (deck, stake) pairs.
- White-stake guarantee: if the final slot is reached with zero whites, it is forced white — every list contains at least one.
- All of it is data-driven (DB tables) so moderators tune weights/bans live, without a deploy.

## Proposal

1. **A reusable weighted-tuple pool helper in the API** (opt-in, e.g. `MPAPI.BanPick.build_weighted_pool(policy)`), implementing the constraints above and returning `{ key, stake }` items. The engine already supports everything downstream of it: pool items may be `{ key, stake, ... }` tables, and `config.decorate_tile(card, item)` renders stake stickers — no engine changes needed.
2. **Policy comes from the consumer, per gamemode** — each mod passes its own policy into `BP.start` via `build_pool`, exactly like `schedule`/`decorate_tile` today. PvP and Speedrun configure independently.
3. **Policy data should be server-deliverable** (gamemode config in the match/lobby payload, keyed by `modId + gameMode`) so allowed lists/weights can be tuned like the bot's DB-backed mults — without shipping mod updates. Client-side defaults as fallback.

## Notes

- Sampling differs today: the in-game default samples decks without replacement (each deck once); tuple pools intentionally allow one deck at multiple stakes. Both remain valid choices per gamemode — the helper covers the tuple case, the current default stays for plain drafts.
- Related: the low-rated-player pool veto (see linked issue) depends on this — its mercy rule ("force white stakes into the reroll") only has meaning against a weighted tuple pool.

Start by locating the ban-pick engine's BP.start entry point, its current default pool, and the build_pool/decorate_tile integration. Use src/utils/TupleBans.ts as the behavioral reference for weighted tuple generation and constraints. Done means consumer policies work per mod and gamemode, with server-deliverable configuration and client defaults as described.

Written by the indexing model from the issue text.