Variants, step 1b: POST /api/v1/feed/tells with variants: 2 starts two builder runs in one group #1644

Second half of #1571.

Done looks like:

• [ ] The tell route in src/http/feed.ts accepts variants, an integer 1–4 validated at the boundary; 0, 9, "2" and 2.5 are a 400 with a message; absent means 1.
• [ ] n > 1 dispatches n builder runs sharing one fresh variant_group; the pull each run opens carries it. n = 1 leaves the group null.
• [ ] Tests (in the new GATE file from step 1a): variants: 2 makes two runs in one group; 0 and 9 refused; a plain tell has no group.
• [ ] No change to how asks group or count pulls (src/web/studio/asks.ts): sibling pulls are two ordinary green pulls to the asks for now; the deck that chooses among them is #1410. Run bun test test/asks*.test.ts before pushing.