Share the duplicated tool helpers and cut four empty claims

[ryanduguid]

What this changes

Tools only; ozzit.xlsx, src/, functions.csv and the AFE store are untouched.

• Deletes test_regression_tests_remain_proportionate_to_production_tools (a test-to-tool line ratio of 0.50 that proves nothing about behaviour and fails on any legitimate trim) and physical_code_lines, which had no other caller.
• The eleven byte-identical text-IO helpers across seven modules become one read_text and write_text pair in tools/sanitise_workbook.py, which the postbuild modules already reach; every copy was checked identical by AST hash before merging.
• The two identical xml_escape bodies become xml.sax.saxutils.escape (fuzzed against the old body on 3,469 inputs; identical for &, <, > and quotes). The copy inside the frozen v3.0.0 provenance rebuild stays.
• column_number was defined twice with identical bodies; one lives in sanitise_workbook.py, which both callers already import (keeping either original would have pulled a mutating tool into a read-only gate or the reverse).
• The ruff-format pre-commit hook goes: ruff format --check would reformat 45 files today and no CI gate checks formatting, so the hook rewrote files nothing verified. CONTRIBUTING says the hooks run the pinned ruff check.
• Prose: docs/australian-modelling.md names the profile (front-loaded, the author's own wording from the lease schedule's help) instead of "the profile AASB 16 is known for" and drops the kicker; docs/workbook-use.md drops the sentence that fits any spreadsheet and keeps the specific volatility facts; CHANGELOG loses two bold kickers and two em dashes; llms.txt opens on the workbook's own facts (138 functions, no VBA or add-ins) instead of the shared template sentence.

Why

Verification

python -m ruff check .                                   # All checks passed!
python -m mypy --config-file mypy.ini                    # Success: no issues found in 27 source files
python tools/verify_workbook.py ozzit.xlsx               # OK: ozzit.xlsx, 138 functions, 169 parts
python tools/verify_sources.py ozzit.xlsx src            # OK: src/ reproduces all 138 functions
python tools/verify_signatures.py src                    # OK: all 130 signatures, 130 parameter tables and 127 example blocks
python tools/verify_previous_names.py functions.csv      # OK: 138 functions, 130 of them replacing a name
python tools/verify_index.py ozzit.xlsx src functions.csv   # OK: all 138 functions.csv rows match
python tools/verify_afe.py ozzit.xlsx src                # OK: AFE store matches all 6 modules
python -m unittest discover -s tools/tests -v            # Ran 216 tests, OK (218 before: the two deleted tests)

Mutation check: stubbing the shared read_text to return an empty string fails 22 postbuild tests plus one error; restored, 42 postbuild tests OK.

Checklist

• No client, taxpayer, employee or payroll data, and no credentials, tokens, tenant IDs or .env contents, appear in the diff, the commit messages or this description.
• The change is single-purpose. (One audit, one repository, batched as agreed.)
• New or changed behaviour has a test that fails without the change. (The mutation check above; the escape fuzz.)
• Documentation, README and any docstring that describes the changed rule were updated together with it.
• Any new dependency is justified in the description, or there is none.

[qodo-code-review]

Code Review by Qodo

🐞 Bugs (0) 📘 Rule violations (0) 📎 Requirement gaps (0)

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