fix(yamlfmt): keep fitting flow collections intact

[felix-windsor]

Closes #623

Summary

• Give an over-width flow collection the extra room from moving below its mapping key before deciding whether to split its elements.
• Keep the collection intact when it fits at the deeper value indentation; collections that are still too wide continue to use the existing element-per-line layout.
• Preserve already-multiline flow collections rather than passing them through the single-line relocation path.
• Add a regression test for the reported sequence-item shape, including a second formatting pass to pin idempotency.
• Document the fix in the changelog.

For the issue reproduction, the formatter now produces the same bytes as Prettier 3:

items:
  - labels:
      ["aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"]
    name: x

Validation

• go vet ./...
• gofmt -s -l -e . — clean
• golangci-lint run ./... — 0 issues
• go generate ./pkg/filetype/...
• go build -o /dev/null ./cmd/cfv
• go test -cover -coverprofile coverage.out ./... — all packages passed, 92.3% total coverage
• npx --yes prettier@3 --parser yaml on the issue reproduction — byte-for-byte match

Implemented and verified with Codex assistance; the final diff and test results were reviewed before submission.