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>

tests/test_collections.c

@@ -1,5 +1,5 @@
 /*
- * List and tree additions -- src/collections.c, TODO.md section 3.6.
+ * List and tree additions -- src/collections.c.
  *
  * The bare-node list functions, the tracked aksl_List container, and the binary
  * search tree. The hash map and string buffer have their own files.
@@ -407,7 +407,7 @@ static akerr_ErrorContext AKERR_NOIGNORE *free_that_fails_once(void *ptr)
  * single walk. Only the first is kept and handed back; the rest have to be
  * released, or a walk over n nodes with a broken free would consume n pool slots
  * and exhaust the pool -- the failure mode the whole pool-accounting section of
- * TODO.md 1.9 exists to catch.
+ * the cross-cutting wrapper contract exists to catch.
  */
 static akerr_ErrorContext AKERR_NOIGNORE *free_that_always_fails(void *ptr)
 {
@@ -800,7 +800,7 @@ static int test_tree_insert_orders_the_leaves(void)
 }
 
 /*
- * TODO.md 2.2.15: aksl_TreeNode.parent was declared and never touched by
+ * aksl_TreeNode.parent was declared and never touched by
  * anything in the library. These are the functions that set it, and
  * aksl_tree_remove is the one that needs it.
  */