Improved readability and minor performance of sanitize_strftime_format regex substitution

[xMintTea]

The current implementation of sanitize_strftime_format uses an inline lambda with indexed groups (r"%s%%0%s%s" % (m[1], mapping[m[2]], m[2])) and a compact regular expression. While correct, it is not immediately obvious what the replacement does and why the groups are referenced by index.

This patch replaces the lambda with a small helper function _replace_specifier, uses a named group (?P<letter>...) in the regex, enables re.VERBOSE for readability, and switches to an f-string for clarity.

No behavioral changes – the early exit condition and the mapping dictionary remain identical. Equivalence was verified on Windows by mocking the early exit and comparing outputs for a comprehensive set of format strings, including edge cases with escaped percent signs (%%). The regex match results were also confirmed to be the same.

Additionally, the use of an f-string and named group yields a ~12-13% performance improvement over the old %-formatting with m[1]/m[2] access (measured with timeit on 240,000 calls).

[Jacob Walls]

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