fix(client): count opponents correctly when the local seat is eliminated

[jeffrey701]

Problem

useResumables (client/src/hooks/useResumables.ts) builds a resumable-match summary and computes:

const liveCount = state.players.filter((p) => !p.is_eliminated).length;
// …
opponentCount: Math.max(0, liveCount - 1),

liveCount already excludes eliminated players, and the - 1 removes the local human (seat 0). But that assumes seat 0 is always alive. In a still-resumable multi-seat / FFA match where seat 0 has been eliminated, seat 0 isn't part of liveCount, so subtracting 1 undercounts the opponents by one. Example — players = [{id:0,eliminated}, {id:1,live}, {id:2,live}] → reports 1 opponent, should be 2.

Fix

Extract the counting into a pure, unit-tested countLiveOpponents helper (client/src/hooks/liveOpponents.ts) that only subtracts the local seat when it is itself alive, and call it from the hook. (Extracting the logic is what makes it testable without standing up the whole async hook — matching how the repo keeps other pure logic, e.g. gridBandMath, in dedicated tested modules.)

Tests

Adds liveOpponents.test.ts: seat 0 alive → live-others count; seat 0 eliminated → not decremented (the old formula returned 1, the fix returns 2 — discriminating); eliminated opponents excluded; degenerate all-eliminated / lone-survivor cases → 0.

Self-contained: only useResumables.ts, a new pure helper, and its test; no engine/Rust/protocol changes.

Model: claude-opus-4-8

[gemini-code-assist]

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