fix(libc): preserve strftime literal conversions - #3653

What

strftime("%Q") currently returns 0 on Windows instead of the literal Q produced by FreeBSD 11.4. A trailing %, incomplete modifiers and repeated E/O or padding modifiers have the same host-dependent problem.

Parse those literals before calling the host formatter, escaping literal percent signs so a following character cannot become a new conversion. Preserve ordinary conversions and the existing %k, %l, %s, %v and %+ expansion. Update TechnicalDebt to retain the remaining padding and timezone gaps.

The behavior follows _fmt in FreeBSD 11.4 strftime.c, including its repeated-modifier and end-of-format branches. %P is unknown in that version and becomes literal P.

Tested

• Windows, MinGW GCC 15.2, Release: full default build and explicit libs target passed.
• The added guest_time_conversion regression fails against unchanged main at %Q and passes with this fix. It covers 27 format cases, exact-fit and undersized buffers, empty formats and a zero-size buffer.
• guest_time_conversion, guest_formatting, guest_wide_formatting, guest_errors, guest_locale and guest_freebsd_clocks passed five consecutive runs each (30 executions).
• External comparison of the built library's two entry points with FreeBSD 11.4's original parser/helpers and C locale: 35,032 formats, 350,320 buffer/alias checks per run, zero mismatches with the reference compiled at -O0 and -O3. Portability shims replace locale lookup, types and host mktime; known padded conversions and standalone %z/%Z are excluded. No reference code or investigation files are added to the repository.
• Reviewed actual library diffs from 209 open PR heads. feat(libc): implement wcsftime #3637 adds wcsftime in the same source file and delegates to the existing strftime; it does not implement this literal-conversion fix. A merge check combines the source automatically; the adjacent TechnicalDebt changes need resolution if both PRs merge. The closed timezone fix fix(libc): preserve guest time zones in Windows strftime #2665 is a separate gap.
• Conventions checker and git diff --check passed. Linux runtime and console/game testing were not performed.

Checklist

• Based on current main; no other open PR implements the same functions
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• Unimplemented paths throw (NotImplemented_nid_no_patch); silent stubs are listed in TechnicalDebt
• No notes, investigation .md files or images added to the repository (attach them to this PR)
• New third-party code is a submodule built from source
• Generic behaviour, not specific to one title
• Depends on: none
• AI-assisted: yes