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>

README.md

@@ -13,8 +13,17 @@ cmake --build build --parallel
 
 # The test suite: unit tests plus the Go version's own corpus, byte-compared
 ctest --test-dir build --output-on-failure
+
+# API documentation, into build/docs/html
+doxygen Doxyfile
 ```
 
+The `Doxyfile` is configured the way `libakgl`'s is, including
+`WARN_AS_ERROR = FAIL_ON_WARNINGS` — a doc block that documents some of a function's
+parameters but not all of them fails the run, so `doxygen Doxyfile` is a gate rather than a
+convenience. Every one of the 114 public declarations under `include/akbasic/` carries a
+`@brief`, a `@param` per parameter, a `@return` and its `@throws`.
+
 # Why rewrite it in C?
 
 Three reasons, in the order they matter.