Document the public API in libakgl's Doxygen style

Adds a Doxyfile in the shape libakgl uses -- fifteen deliberate lines, including
WARN_IF_UNDOCUMENTED and WARN_AS_ERROR=FAIL_ON_WARNINGS -- and fills in the 70
public declarations that had no doc block, bringing include/akbasic to 114 of
114.

libakgl is the model rather than libakstdlib. Measured before starting: libakgl
documents 122 of 122 public declarations and libakstdlib 3 of 25, and
libakstdlib's Doxyfile is the unedited doxygen default, PROJECT_NAME = "My
Project" and an empty INPUT. So the house standard is libakgl's, down to the
boilerplate phrasing for the recurring parameters -- "Object to initialize,
inspect, or modify", "Output destination populated by the function", "`NULL` on
success, otherwise an error context owned by the caller".

Worth knowing what the gate actually gates. EXTRACT_ALL=YES suppresses
doxygen's undocumented-entity warnings, so the rule it enforces is that a
*partial* block is an error: document one @param and you must document them all.
Verified by deleting a @param and confirming a non-zero exit, then restoring it.
Full coverage is therefore a convention this commit adopts rather than something
the tool made me do.

Where a contract is non-obvious the block says so rather than restating the
signature: math_plus explains why it alone mutates its left operand, new_unary
notes that hanging the operand on .right is what makes the parser miscount a
negative literal argument, leaf_to_string warns that an assignment renders with
an empty operator, and stop_waiting records that a verb nobody is waiting for is
tolerated. Each cross-references its TODO.md section 6 item.

Also records in TODO.md that this repository has no CI, which libakgl and
libakstdlib both have -- so ctest, the sanitizer build, coverage and this new
doxygen gate are all run by hand today.

ctest 61/61; doxygen Doxyfile exits 0; no warnings under -Wall -Wextra.

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

Doxyfile

@@ -0,0 +1,14 @@
+PROJECT_NAME           = libakbasic
+PROJECT_BRIEF          = "Commodore/Dartmouth BASIC interpreter, embeddable in C"
+OUTPUT_DIRECTORY       = build/docs
+INPUT                  = include/akbasic src examples
+FILE_PATTERNS          = *.h *.c
+RECURSIVE              = YES
+EXTRACT_ALL            = YES
+EXTRACT_STATIC         = YES
+WARN_IF_UNDOCUMENTED   = YES
+WARN_IF_DOC_ERROR      = YES
+WARN_AS_ERROR          = FAIL_ON_WARNINGS
+GENERATE_HTML          = YES
+GENERATE_LATEX         = NO
+QUIET                  = YES