Repoint the TODO.md citations that no longer resolve

Its own SS0-SS9 structure survived the move to the tracker, so 133 citations
still resolve and are left alone. Thirteen did not.

Six named another repository's TODO.md by a section number: libakstdlib 1.6,
2.2.2 and 2.3, and libakgl's file. Those repositories dropped their numbering
entirely, so each now names UPGRADING.md, an issue, or the tracker.

Three cited 'TODO.md section 12', which has never existed here -- the defect
list is SS6. TODO.md itself caught two others of that class earlier. The
CMakeLists comment carried a stale premise with it: eleven defects
'deliberately reproduced and not yet fixed' stopped being the rule when SS0.1
retired the fidelity constraint.

Four told a reader to record work in TODO.md; the scanner defect is issue #4,
the audio_tables mutation gap is #25, and the UI gaps are libakgl #79 and #80.

Verified: cmake --build build && ctest --test-dir build, 112/112.

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

src/symtab.c

@@ -15,7 +15,7 @@
  * Probe for `key`. On success *slot points at either the slot holding the key or
  * the first free slot it could occupy; *found says which. Uses
  * aksl_strhash_djb2 rather than a private hash. That wrapper sign-extends char,
- * so a high-bit byte hashes wrong (deps/libakstdlib/TODO.md 1.6) -- harmless
+ * so a high-bit byte hashes wrong (libakstdlib's UPGRADING.md) -- harmless
  * here because BASIC identifiers are 7-bit ASCII, and it would only ever cost
  * probe efficiency, never correctness, since the key comparison is a strcmp.
  */