perf(ccusage): avoid Rust dedupe key allocations

Replace per-entry messageId/requestId string concatenation with a hashed lookup and collision-safe string comparison. This keeps duplicate replacement semantics unchanged while avoiding allocation of a combined key for every usage entry during global deduplication.

Also rename the Rust color helper from colour/Colour to color/Color so the existing spell-check workflow passes on this branch.

The rejected alternatives were opt-level=s, which increased binary size from 757024 to 854416 bytes without a speed win, and a manual JSON fast path, which slowed daily runs. Baseline/current JSON output matched for daily, session, and blocks.

Author: ryoppippi