Split the finder into its stages

[Komoszek]

Stacked on #67; retarget to main once it merges. #68 then lands on top of this, so the transitive rounds arrive as a change inside settler.dart rather than a rewrite of finder.dart. A move, not a change: same tests (278), same --verbose narration.

Ciach was ~1000 lines with five jobs. It now keeps the server session and the pipeline (discover → collect → fetch → settle), ~200 lines; each stage is a collaborator, in the pattern RemoveSafety, OverrideRemovals and CrossLibraryReferences already follow.

Module	Lines	Job
candidate_collector.dart	292	which declarations get checked: the symbol walk, option filters, entry points and their containers
reference_fetch.dart	152	the server round trips — references, tokens, syntax nodes — into SourceIndex
settler.dart	312	from references to sorted findings: classify, conventions, remove-safety, override coupling
verdict.dart	204	what becomes of one unused candidate — suppressed, report-only, or a finding — and its hints

Code moved verbatim except: _toUnused → Verdict.finding, _byLocation → top-level compareByLocation, _semanticTokensOrEmpty → top-level (the collector and the fetch both need it), and SuperclassChecks/OverrideRemovals are built once in Settler.settle instead of inline.

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https://claude.ai/code/session_011fqaUyaQtY7AkuTYBjAUJv