Drop the TODO.md section numbers from 26 files

Eighty-five comments cited section numbers -- 1.1, 2.2.6, 3.6 -- from a
numbering the file had already abandoned before the move to the tracker. They
label completed work, so the pointer was the only wrong part.

The citation is removed and the sentence kept, which is what issue #27
recommended: these are labels, not references, and a label carrying a
version-dependent pointer goes stale again at the next reorganisation. Where a
pointer earns its place it names what actually holds the content now --
UPGRADING.md for the confirmed defects, libakerror #15 for the target
namespacing, issue #7 for the mutation survivors.

README.md and akstdlib.h sent readers to TODO.md for 'what is still open';
they name the tracker.

Verified: cmake --build build && ctest --test-dir build, 19/19.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

src/string.c

@@ -1,12 +1,12 @@
 /*
- * string.h wrappers -- TODO.md section 3.1.
+ * string.h wrappers.
  *
  * This is the section akbasic needed most and could not have: across its source
  * it calls strlen 37 times, strcmp 16, strncpy 15 and strstr once, every one of
  * them raw because there was nothing here to call instead. Ten of those sites
  * are the same idiom written out by hand -- a length check, then strncpy, then
  * an explicit NUL -- which is exactly the "truncation reported as an error
- * rather than silently accepted" that TODO.md 3.1 asks for.
+ * rather than silently accepted" that the wrapper contract asks for.
  *
  * Two conventions run through the whole file.
  *