check_doc_claims.py: 25 surviving mutants in the canonical-source helpers

[cdeust]

What

A scoped mutation run over scripts/check_doc_claims.py leaves 25 surviving
mutants, all in the canonical-source helpers, none in the scan/report path:

Function	Survivors
canonical_reference_count	10
canonical_tool_counts	7
canonical_version	3
canonical_mechanism_count	3
read	2

Reproduce

scripts/mutation_check.sh tests_py/scripts/test_check_doc_claims.py scripts/check_doc_claims.py

Why it is filed rather than fixed

These are pre-existing gaps in functions the exemption-registry change does not
modify. That change's own surface — scannable_lines, exemption_registry,
scan_claims, check_counts, collect_failures — is at 0 survivors, which
is the §12.4 standard for changed code. Closing the remaining 25 means writing
new tests for four untouched helpers, which is its own unit of work rather than
a rider on a gate fix (§14.3, §15.1 sequencing).

Note this run was only possible after that PR made scripts/ reachable by the
mutation runner at all — previously source_paths was mcp_server-only, so
these survivors had never been measured.

Acceptance criteria

• 0 surviving non-equivalent mutants in scripts/check_doc_claims.py.
• Any mutant judged equivalent is documented with a written rationale at the
  use site or in the run notes (§12.1) — "ignore" is not an option.
• canonical_reference_count's entry filter is pinned against a bibliography
  containing headings, separators and blank lines (partially covered by
  test_reference_entries_exclude_headings_and_separators).