Read the keyboard: implement GET, GETKEY and SCNCLR

The poll_key half of group E, against the akbasic_InputBackend record. The
libakgl implementation behind it is akgl_controller_poll_key(), which drains a
ring the library fills from SDL events the host pumps -- so the interpreter can
answer "is there a key waiting" without owning an event loop, which is what goal
3 requires.

GET and GETKEY differ in one way and it is the interesting one. GET takes
whatever is there including nothing, and an empty buffer is success with the
empty string rather than an error -- that is what happens on most iterations of
every GET loop ever written, and upstream is explicit that its poll reports it
the same way. GETKEY waits, and since the library may not block, waiting is
spelled as holding the step loop: the verb sets a flag and akbasic_runtime_step()
declines to advance until a key arrives. Every step still returns and a bounded
run() still comes back, so a host keeps its frame rate; the program simply does
not move past the GETKEY.

The PLAY queue is serviced before that check on purpose -- music should keep
playing while a program waits for a keypress. Withdrawing the input device while
a GETKEY is holding releases it rather than wedging the script on a device that
no longer exists.

Both verbs accept an integer variable as well as a string one and give it the raw
key code, which is what a program testing for cursor or function keys needs. No
key is code zero, matching what a C128 reports. A float variable is refused: it
is neither a character nor a code.

SCNCLR goes through the text sink rather than a device, because the sink is where
PRINT already goes and is the only thing that knows what a screen means for this
host. The stdio sink treats it as a no-op; clearing a pipe means nothing.

One thing worth knowing before writing any bounded-run test, and now commented in
tests/input_verbs.c: source lines are stored indexed by line number and the
cursor starts at zero, so a step is spent on each empty slot along the way. A
program at line 10 needs eleven steps before it has run anything.

70/70 ctest, clean under -Wall -Wextra, doxygen clean.

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

TODO.md

@@ -424,12 +424,13 @@ Order by what unblocks the most, and by what does not need `libakgl` to grow fir
 | B. Housekeeping | `NEW`, `CLR`, `CONT`, `RESTORE`, `RENUMBER`, `SWAP`, `TRON`, `TROFF`, `HELP` | nothing |
 | C. Errors | `TRAP`, `RESUME`, `ER`, `ERR` | needs a BASIC-visible error object |
 | D. Strings/format | `USING`, `PUDEF`, `WIDTH`, `CHAR` | nothing |
-| E. Console | `GET`, `GETKEY`, `SCNCLR`, `WINDOW`, `KEY`, `SLEEP`, `WAIT`, `TI` | `GET`/`GETKEY`/`SCNCLR` nothing; the rest want the sink or a clock |
+| E. Console | `WINDOW`, `KEY`, `SLEEP`, `WAIT`, `TI` | the sink, or the clock `akbasic_runtime_settime()` now carries |
 | F. Disk | `DOPEN`, `DCLOSE`, `APPEND`, `RECORD`, `HEADER`, `COLLECT`, `BACKUP`, `COPY`, `CONCAT`, `RENAME`, `SCRATCH`, `DIRECTORY`, `CATALOG`, `DCLEAR`, `DVERIFY`, `SAVE`, `LOAD`, `VERIFY`, `BLOAD`, `BSAVE`, `BOOT` | `aksl_f*` only; no new akgl |
-| G. Graphics | `GRAPHIC`, `DRAW`, `BOX`, `CIRCLE`, `PAINT`, `COLOR`, `SCALE`, `SSHAPE`, `GSHAPE`, `LOCATE` | nothing — `akgl_draw_*` landed at 42b60f7 |
+| ~~G. Graphics~~ | ~~`GRAPHIC`, `DRAW`, `BOX`, `CIRCLE`, `PAINT`, `COLOR`, `SCALE`, `SSHAPE`, `GSHAPE`, `LOCATE`~~ | **done** — `src/runtime_graphics.c`, `tests/graphics_verbs.c` |
 | H. Sprites | `SPRITE`, `MOVSPR`, `SPRCOLOR`, `SPRDEF`, `SPRSAV`, `COLLISION` | `libakgl` sprite/actor API — check before filing |
-| I. Audio | `PLAY`, `SOUND`, `ENVELOPE`, `VOL`, `TEMPO` | nothing — `akgl_audio_*` landed at 42b60f7 |
-| I′. Audio, still gapped | `FILTER`, `SOUND`'s sweep arguments | `libakgl` has neither; see §7 |
+| ~~I. Audio~~ | ~~`PLAY`, `SOUND`, `ENVELOPE`, `VOL`, `TEMPO`~~ | **done** — `src/runtime_audio.c`, `src/play.c`, `tests/audio_verbs.c` |
+| I′. Audio, still gapped | `FILTER`, `SOUND`'s sweep arguments | `libakgl` has neither; both refused with `AKBASIC_ERR_DEVICE`, both filed in §7 |
+| ~~E′. Console input~~ | ~~`GET`, `GETKEY`, `SCNCLR`~~ | **done** — `src/runtime_input.c`, `tests/input_verbs.c` |
 | J. Machine | `SYS`, `FETCH`, `STASH`, `POKE`/`PEEK` variants | nothing |
 
 `LOCATE` used to appear in both E and G. It belongs to G alone: in BASIC 7.0 `LOCATE` moves the
@@ -552,6 +553,21 @@ behaviour to port. They matter to somebody typing in a listing out of a C128 man
     default-tempo quarter note at 500 ms — 120 beats per minute. If the real constant turns
     up, that is the one line to change.
 
+23. **`GETKEY` holds the step loop rather than blocking.** A C128 sits on the keyboard inside
+    `GETKEY` until somebody types. §1.6 forbids the library blocking, so the verb sets a flag
+    and `akbasic_runtime_step()` declines to advance the program until a key arrives. Every
+    step still returns and a bounded `akbasic_runtime_run()` still comes back, so a host keeps
+    its frame rate; the program simply does not move past the `GETKEY`, which is the part a
+    program author cares about. The `PLAY` queue is serviced *before* this check on purpose —
+    music should keep playing while a program waits for a keypress. Withdrawing the input
+    device while a `GETKEY` is holding releases it rather than wedging the script forever.
+
+24. **`GET` and `GETKEY` into a numeric variable yield the key code.** BASIC 7.0's `GET` takes
+    a string variable. Accepting an integer one as well, and giving it the raw code, is what a
+    program testing for cursor or function keys needs and costs nothing. No key is code zero,
+    matching what a C128 reports. A float variable is refused: it is neither a character nor a
+    code.
+
 ---
 
 ## 6. Reference defects — ported faithfully, fix them deliberately