Port the BASIC interpreter from Go to C

Reproduces deps/basicinterpret in C, in the idiom of the ak* libraries. All 41
.bas files in the reference's corpus produce byte-identical stdout, including
the trailing double newline on an error line -- that comes from basicError
building a string ending in \n and handing it to Println, and
array_outofbounds.txt encodes it.

The corpus is driven in place from the submodule as 41 individual CTest cases
rather than copied, so it cannot drift from upstream. Eighteen unit tests cover
what the corpus cannot reach.

Three structural changes carry most of the work. Go's three reflection lookups
(Command*, Function*, ParseCommand*) become one sorted dispatch table in
src/verbs.c searched with bsearch; adding a verb is a row and two functions. The
five Go maps become one fixed open-addressed table over aksl_strhash_djb2. And
run(), which owned the process until MODE_QUIT, splits into step() plus a
bounded run() -- goal 3 requires a host game to be able to bound execution, and
nothing in the library now terminates the process or touches SDL.

Output goes through an akbasic_TextSink vtable. src/sink_stdio.c is what makes
the corpus runnable with no SDL present; the akgl-backed sink is still to come
and is blocked on libakgl having no text-measurement call.

src/convert.c exists because libakstdlib's aksl_ato* family cannot report a
conversion failure (its TODO.md 2.1.5). The reference checks strconv's error at
four sites and turns it into a BASIC error; routing those through aksl_atoi
would have turned four diagnosable errors into wrong answers, with VAL("garbage")
quietly returning 0. TODO.md 1.9 records which libakstdlib calls are cleared for
use here and which are not.

Reference defects are reproduced, not fixed: the golden files encode the observed
behaviour and a silent correction is a behaviour change. TODO.md section 6 lists
sixteen, and tests/known_reference_defects.c asserts the *correct* contract for
six of them under AKBASIC_KNOWN_FAILING_TESTS, so a fix shows up as
"unexpectedly passed". Five of the sixteen were found by this port and are new:
subtraction stops after one operator so 1-2-3 computes 1-2 and abandons the rest
of the line (a wrong answer, not a refused one); a unary-minus argument inflates
a function's arity so ABS(-9) is rejected; a comparison operator in a line's
final column is dropped; hex literals never survive the scanner; and the
"Reserved word in variable name" check is dead code.

Where the reference reaches undefined behaviour by a route that is defined in Go
-- an out-of-range shift, a negative string multiplier, integer division by zero
-- this raises instead of inheriting the UB. No golden case exercises any of
them.

The top-level CMakeLists shadows add_test, set_tests_properties and
add_custom_target around all three add_subdirectory calls. Without it libakerror's
tests land in our suite as Not Run, and its un-namespaced `coverage` target stops
a coverage build from configuring at all. Test targets are akbasic_test_<name>:
bare test_<name> collides with libakstdlib's, which is what broke libakgl's
configure in c2b16d3.

ctest 59/59; ASan+UBSan 59/59; 92.3% line and 96.9% function coverage; no
warnings under -Wall -Wextra. Branch coverage is not a target, for the reason
libakstdlib and libakgl both record: the akerror macros expand into large branch
trees at every call site.

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

include/akbasic/error.h

@@ -0,0 +1,71 @@
+/**
+ * @file error.h
+ * @brief Declares akbasic's status codes and the registry claim for them.
+ */
+
+#ifndef _AKBASIC_ERROR_H_
+#define _AKBASIC_ERROR_H_
+
+#include <akerror.h>
+
+/*
+ * libakerror 1.0.0 is the floor. That release moved the status-name table into a
+ * private registry -- AKERR_MAX_ERR_VALUE and __AKERR_ERROR_NAMES are gone, the
+ * registry entry points raise akerr_ErrorContext * instead of returning int, and
+ * the library gained an soname -- so a translation unit that pairs this header
+ * with a pre-1.0.0 akerror.h is an ABI mismatch, not just a compile problem.
+ *
+ * libakerror publishes no version macro, so this feature-tests on
+ * AKERR_FIRST_CONSUMER_STATUS, which that release introduced. Same guard
+ * libakstdlib and libakgl already carry.
+ */
+#ifndef AKERR_FIRST_CONSUMER_STATUS
+#error "libakbasic requires libakerror >= 1.0.0: the akerror.h on the include path predates the status registry. Rebuild and reinstall libakerror."
+#endif
+
+/*
+ * libakerror reserves 0-255 for the host's errno values and its own AKERR_*
+ * codes. libakgl claims 256-260. akbasic claims 512-767, leaving 261-511 as
+ * headroom for libakgl to grow into -- see CLAUDE.md for the coordinated range
+ * map, which is the only coordination there is: libakerror can enumerate its
+ * consumers no better than we can.
+ *
+ * These are absolute constants, never offsets from AKERR_LAST_ERRNO_VALUE, so a
+ * libc that grows an errno cannot move them. An enum rather than a chain of
+ * #defines so the values stay compile-time integer constants -- HANDLE expands
+ * to case labels, which require that.
+ *
+ * Add a code here and you must name it in akbasic_error_register(), or it prints
+ * as "Unknown Error" in every stack trace that carries it.
+ */
+#define AKBASIC_OWNER		"akbasic"
+
+enum {
+    AKBASIC_ERR_BASE	= 512,	/** Start of akbasic's reserved status range */
+    AKBASIC_ERR_SYNTAX	= AKBASIC_ERR_BASE,
+				/** Parse-time grammar violation */
+    AKBASIC_ERR_TYPE,		/** Incompatible types in an operation */
+    AKBASIC_ERR_UNDEFINED,	/** Reference to an undefined verb, function or label */
+    AKBASIC_ERR_BOUNDS,		/** Array subscript or pool index out of range */
+    AKBASIC_ERR_ENVIRONMENT,	/** Environment pool exhausted or orphaned environment */
+    AKBASIC_ERR_VALUE,		/** A value was malformed, truncated or unconvertible */
+    AKBASIC_ERR_STATE,		/** A verb ran outside the block structure it requires */
+    AKBASIC_ERR_LIMIT	= AKBASIC_ERR_BASE + 256
+};
+
+/**
+ * @brief Claim the akbasic status band and register a name for every code in it.
+ *
+ * Reserves the whole 256-value range in one call -- libakerror treats only an
+ * identical reservation as a repeat, so a subset or superset raises -- and then
+ * names each code through the owned entry point. Call it before anything else in
+ * the library. Repeating the call is a no-op.
+ *
+ * @return `NULL` on success, otherwise an error context owned by the caller.
+ * @throws AKERR_STATUS_RANGE_OVERLAP When another component already owns part of the band.
+ * @throws AKERR_STATUS_RANGE_FULL When libakerror has no reservation slots left.
+ * @throws AKERR_STATUS_NAME_FULL When libakerror's name registry is full.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_error_register(void);
+
+#endif // _AKBASIC_ERROR_H_