refactor(complexity): phase 3 — the goast provider methods - #315

Third phase of the complexity plan (#312): nine functions under the
thresholds by behavior-preserving decomposition, including the repo's
worst. ConstGroups (70) becomes a named state machine —
constGroupsFromDecl with an explicit flush closure, entries via
constEntriesFromSpec, its anonymous local types promoted to file-level
constEntry/constGroup. Structs (49) extracts
structFields/fieldDetailsFrom; Callable (49) and TypeDoc (24) share
typeSpecDoc via per-file finders; Methods (38) splits filter from
construction; SortDeclarations (24) splits block collection from
splicing; Calls (22) and Composites (35) extract their closure bodies;
spacingRulesFromConfig (17 gocyclo) goes table-driven over its
order-independent fields.

The plan's pre-declared structure question is now concrete and recorded
in the phase notes: the collectGoFiles+parseFile iteration remains
repeated in five methods; a shared per-file walker is available for a
ruling, not adopted unilaterally.

Zero complexity findings remain in the file; no new findings anywhere;
make vet and full make test pass unmodified; gofmt clean.
Plan: docs/plans/complexity-refactors.md (phase 3 complete).