Retire the byte-for-byte fidelity constraint, and fix what it was blocking

The Go reference is deprecated and will not be updated, so the two projects
are no longer required to match. Recorded as TODO.md section 0.1, first thing
in the file, because it silently reverses the premise several later sections
were written on -- an agent reading section 6 without it will park work that
is no longer blocked.

Section 6 item 16 was the item waiting on exactly this and is now fixed: the
keyword tables are searched on the base name with any type suffix stripped, so
PRINT$, LEN# and GOTO% are refused as variable names and the reference's own
diagnostic stops being dead code. It cost the one golden case predicted, and
the cost was nothing -- renaming a variable in strreverse.bas left its expected
output byte-for-byte unchanged.

AKBASIC_KNOWN_FAILING_TESTS is empty as a result and known_reference_defects.c
is gone. Section 6 items 1-9 and 11 are reopened as an ordinary defect list.

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

tests/reference/README.md

@@ -14,36 +14,49 @@ interpreter was ported from:
 They are registered as the `golden_*` CTest cases and run against `basic` on every build, in
 both the default and the `-DAKBASIC_WITH_AKGL=ON` configurations.
 
-## Why they are a copy now, when they deliberately were not
+## Why they are a copy, when they deliberately were not
 
 Until 2026-07-31 these were driven *in place* out of `deps/basicinterpret`, and the reasoning
 was written down in `CMakeLists.txt`: the corpus is a submodule, and copying a submodule's
 corpus guarantees drift.
 
-That reasoning was sound. It was overruled on purpose, because the Go dependency is being
-deprecated and a build that cannot run its own acceptance suite without cloning the
-implementation it replaced is not finished.
+That reasoning was sound and it stopped applying. The Go project is deprecated and will not be
+updated, so there is nothing left to drift *from* — and a build that cannot run its own
+acceptance suite without cloning the implementation it replaced is not finished.
 
-**What the change costs, stated plainly so nobody has to rediscover it:** if the Go
-implementation ever gains a test, this directory will not. There is no longer anything
-watching for that. The trade is deliberate — the Go project is the thing being retired, so a
-corpus that stops tracking it is a corpus that stopped tracking something that stopped moving.
+## What these are for now
+
+**A regression suite, not a specification.** That distinction is the whole of how to treat
+them, and it changed on 2026-07-31 (`TODO.md` §0.1): matching the Go implementation is no
+longer a goal, so a difference between it and this interpreter is not automatically a bug in
+this one.
+
+What they are still good for is unchanged and is worth a lot: forty-one real BASIC programs
+with known-good output, covering arithmetic, arrays, flow control, every builtin function,
+`READ`/`DATA`, labels and three worked examples. That catches regressions whatever its
+provenance.
 
 ## The rule for editing these
 
-**Don't.**
+**Diverge deliberately, never by accident.**
 
-This directory is a *record of what the Go implementation did*, and it is what makes the claim
-"reproduces the reference byte for byte" checkable at all. A failure here means this
-interpreter's behaviour changed, and the answer is to change this interpreter or to record a
-deliberate deviation in `TODO.md` §5 — never to edit the expectation to match.
+A failure here means this interpreter's behaviour changed. Nine times in ten that is a bug you
+just introduced and the answer is to fix the code. The tenth time it is an improvement on the
+reference, and then the answer is:
 
-New cases for this project's own behaviour go in `tests/language/`, which is registered under
-`local_*` for exactly this reason: the prefix says which corpus a failure came from, and a diff
-that touches `tests/reference/` stands out as the thing it is.
+1. Change the expectation in the same commit as the code.
+2. Say why in the commit message.
+3. Add a line to `TODO.md` §5, which is the list of deliberate deviations.
 
-There is one case worth knowing about before anybody "fixes" it.
-`examples/strreverse.bas` names a variable `INPUT$`, which a real Commodore 128 would refuse
-and which this interpreter accepts only because the reference's reserved-word check never
-fires. That is `TODO.md` §6 item 16, and it is the reason that item is filed as a decision
-rather than a defect: making the check work costs this case.
+What is not acceptable is editing a `.txt` to make a red suite green without deciding which of
+the two you are doing.
+
+New cases for this project's own behaviour go in `tests/language/`, registered under `local_*`.
+The prefix says which corpus a failure came from, and a diff touching `tests/reference/` stands
+out as the thing it is.
+
+## Divergences so far
+
+| Case | Change | Why |
+|---|---|---|
+| `examples/strreverse.bas` | Variable `INPUT$` renamed to `SOURCE$` | `INPUT$` is a reserved word with a type suffix, which a real C128 refuses and which this interpreter now refuses too (`TODO.md` §6 item 16). The reference accepted it only because its own reserved-word check never fired. The expectation is byte-for-byte unchanged — the program still prints `REVERSED: OLLEH` — so the case keeps all of its value. |

tests/reference/examples/strreverse.bas

@@ -1,7 +1,7 @@
-10 INPUT$ = "HELLO"
-20 STRLEN# = LEN(INPUT$) - 1
+10 SOURCE$ = "HELLO"
+20 STRLEN# = LEN(SOURCE$) - 1
 30 REV$ = ""
 40 FOR I# = STRLEN# TO 0 STEP -1
-50   REV$ = REV$ + MID(INPUT$, I#, 1)
+50   REV$ = REV$ + MID(SOURCE$, I#, 1)
 60 NEXT I#
 70 PRINT "REVERSED: " + REV$