Fix: Implement bracket-aware custom slicing mode for M4 (#2204)

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Resolves #2204

Root Cause

1. Mode A (Greedy Labels) fails because M4 macros do not implicitly terminate when the next one starts, causing empty stubs (like AC_PROG_F77) to swallow 1,532 LOC of unrelated shell code.
2. Mode B (Lisp Braces) fails because GitGalaxy aggressively shields string literals to prevent brace-depth corruption. Because M4 heavily embeds bash fragments with scattered and unbalanced quotes, the shielding pass shreds the M4 code stream, yielding 0% extraction.

The Fix

Implemented a dedicated custom integration loop (_slice_by_m4_brackets, Mode F) tailored for M4's lexical structure:

• Locates func_start on the unshielded source code.
• Steps forward to track the depth of the outer ( and ).
• Simultaneously tracks M4's bracket quotes [ and ]. Any parenthesis that appears while bracket depth > 0 is completely ignored, correctly shielding unbalanced parenthesis inside bash strings without stripping the strings.
• Added strict structural test harness coverage for these unbalanced-quote edge cases.

This strictly binds macros to their correct AC_DEFUN invocation bounds, completely eliminating the false positive LOC bleeds.

Golden Master Audits

• 157 precise fixes committed to the golden master.
• M4 gross impact LOC perfectly corrected (e.g. AC_PROG_F77 drops exactly from 1532 -> 1).

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