feat(plugin-search): add optional fuzzy matching to findSearchMatches

[rizgan]

What

Adds an opt-in fuzzy option to findSearchMatches in @floatboat/nexus-plugin-search.
Implements roadmap item #17 (Fuzzy search, P2).

When { fuzzy: true } is passed, each line is matched as a subsequence of
the query and annotated with a relevance score, so callers can rank results.

Why

Exact and regex search require the query to appear contiguously. Fuzzy
matching lets users find lines by typing a loose, ordered subset of the
characters — the same interaction model as a command palette / file finder.

Design

• Per-line matching. Each line is treated as a candidate, so a match's
  from/to stays bounded to a single line instead of spanning the whole
  document.
• Greedy left-most subsequence with a small scoring function
  (contiguous runs, word-start hits, and a leading match are rewarded; gaps
  are penalized). Scoring style mirrors the existing slash-state.ts ranking.
• Query characters are always literal (never regex) in fuzzy mode.
• Results stay in document order; sorting by score is left to the caller.

Scope / non-goals

• Purely additive and opt-in — default, regex, whole-word, and case-sensitive
  behavior are unchanged.
• replaceAllMatches, the CM6 search panel, and search history are untouched.
• No new dependencies.

Tests

Added 8 unit tests (fuzzy vs exact, out-of-order rejection, document order,
case sensitivity, literal-not-regex, empty query, score ranking, and "no
score on exact matches"). Full suite: 37/37 passing; package typecheck clean.

[CLAassistant]

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