feat(fuzzy): boost shared-prefix candidates in ranking

[fohara]

Ranking tweak: candidates sharing the query's leading characters get a small score boost (typos cluster at the end of task IDs, so a shared prefix is a stronger signal than raw edit distance alone).

Note: this PR doubles as the first live validation of the new mutation gate (#66) on a source change — the check should surface surviving mutants for the unasserted boost condition. Depending on the verdict this either gets tests added or gets closed as a dogfood probe.

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[fohara]

The mutation gate's verdict on this change: 0.0% diff score — both mutants in the boost condition survived (run, inline annotations at fuzzy.rs:123). Nothing pins !prefix.is_empty() or the &&, i.e. the boost is entirely untested. Withdrawing rather than landing untested ranking behavior — exactly the outcome the report-only gate exists to surface. (This PR was also the gate's first live validation on a source change.)