fix(verify-layout): escape the docstring's LaTeX macro so the tool stops emitting SyntaxWarning (#480)

`tools/verify_layout.py`'s module docstring explains which documents trigger
the Poppler `-bbox` crash, and names the macro whose absence causes it:
`\hypersetup{pdftitle=...}`. In a non-raw docstring `\h` is an invalid escape
sequence, so compiling the module prints

    SyntaxWarning: "\h" is an invalid escape sequence.
    Such sequences will not work in the future. Did you mean "\\h"?

on Python 3.12+ (CPython gh-98401 promoted the long-standing
DeprecationWarning to a SyntaxWarning). The 3.15 language reference still
documents the sequence as a SyntaxWarning, with a SyntaxError only "in a
future Python version".

This is on the real path, not a constructed one: /apply Step 5b runs
`python3 tools/verify_layout.py <pdf>` once per document, so the first
invocation after any source change prints the warning into the middle of the
layout report a user is reading to judge their CV. It is also visible in this
repo's own CI log, where `python -m unittest discover -s tests -t . -v` prints
it above the test results.

Reproduce on master:

    python3 -W error::SyntaxWarning -c "compile(open('tools/verify_layout.py',
      encoding='utf-8').read(), 'tools/verify_layout.py', 'exec')"

raises on master; after this change it is silent.

Escaping (rather than making the docstring raw) keeps the surrounding prose
unchanged, and the single-backtick markup matches the other inline literals in
the same docstring. The rendered text still reads `\hypersetup{...}`.

The new `ToolsCompileWithoutWarnings` case is a pure `compile()` of the source
text of every `tools/*.py` inside `warnings.catch_warnings(record=True)` and
asserts no `SyntaxWarning` or `DeprecationWarning` (3.10 and 3.11 report the
same escape as a DeprecationWarning) - no cache file, no temp file, so it runs
the same on Windows (py_compile's atomic rename over an open
NamedTemporaryFile is what the first version tripped on). It fails on master
and passes here on every Python in the CI matrix, and it guards the next
docstring that quotes a macro.

Verification on Python 3.10, 3.11, 3.12, 3.13 and 3.14: `python3 -m unittest discover -s
tests -t .` (488 tests OK; master is 487), `python3 -W error::SyntaxWarning`
compile of every `tools/*.py`, `tests/test_changelog_structure.py`.

tools/verify_layout.py

@@ -32,7 +32,7 @@ than inventing a layout failure. A broken extractor has two distinct causes: an
 all, and Poppler 26.0x before 26.05 aborts `-bbox`/`-bbox-layout`/`-htmlmeta` on a PDF
 whose Info dictionary carries an empty string in any field - which `hyperref` writes for
 every field it does not set, so any `lualatex`/`pdflatex` document built with `hyperref`
-and no `\hypersetup{pdftitle=...}` triggers a real Poppler crashing on a legal PDF (#451).
+and no `\\hypersetup{pdftitle=...}` triggers a real Poppler crashing on a legal PDF (#451).
 Line height serves
 as a font-size proxy to spot section headings; left edge (xMin) separates bullet lines
 from entry headers.