Give BASIC menus, dialogs and HUD labels over libakgl's UI helpers

Group K, and the first verbs to reach the akgl_ui subsystem 0.9.0 brought
in: MENU and GETMENU and RMENU, DIALOG, HUD and UISTYLE. A program that
wanted a title screen had to draw one out of CHAR and GETKEY, which is
what both breakout tutorials make a reader do.

The interesting part is the impedance mismatch. libakgl's UI is immediate
mode -- widgets are re-declared inside a frame bracket every frame and
clay borrows their text until the bracket closes -- and a BASIC program
says MENU 1, "START" on line 100 and expects it up on line 900, several
hundred frames later. So src/ui_akgl.c is retained on this side and
immediate on that one: the record's entry points are setters that copy
into akbasic_AkglUi, and akbasic_ui_akgl_render() replays the whole set
once a frame from the host's pump. No BASIC string, which lives in the
per-line value pool, is ever what clay is handed.

The shapes are borrowed rather than invented. MENU retires the way SOLID
does -- no entries retires one, no arguments retire them all. GETMENU
holds the step loop the way GETKEY does, so parking is not blocking: the
step still returns, the host keeps its frame rate, and the sprite, audio
and collision services keep running underneath because they run before
the blocking checks. RMENU(n,1) reads and clears the way BUMP() does.
Withdrawing the device or retiring the menu releases a holding GETMENU
with 0 rather than wedging the script, which is akbasic_input_service()'s
rule for a withdrawn keyboard.

One thing a program has to know, and docs/19-user-interface.md says it
twice: a menu that is up owns the cursor keys and Return. It has to, and
retiring it gives them back -- forget the MENU n before an INPUT and the
INPUT never sees the Return that ends it.

akbasic_runtime_set_ui() is its own function rather than a fifth argument
to akbasic_runtime_set_devices(), whose signature has twenty-eight call
sites in tests and documentation that are about something else.

deps/libakgl is not touched. akgl_UiAnchor has the four corners and dead
centre, so HUD offers exactly those five; TODO.md records what a
top-centre and bottom-centre would cost upstream, along with the three
other things this deliberately leaves out. No new error code either --
DEVICE, BOUNDS, SYNTAX and TYPE cover the group, and 520 stays free.

tools/screenshot.c had to learn that "needs a font" and "draws the text
grid" are two questions. They were one, and a UI figure came out black:
the text layer owns every pixel of the rows it covers and painted over
the widgets. The new ui=1 fence attribute asks for the first without the
second; MAINTENANCE.md documents it.

112/112 in both configurations, 112/112 under ASan and UBSan, coverage
94.1% against the 90% gate with src/runtime_ui.c at 99% of lines and
100% of functions, doxygen clean, and the four new figures byte-identical
on a re-render. TODO.md section 8's gate table was stale on several
counts besides these and is refreshed with measured numbers.

Co-Authored-By: Tachikoma (Claude Code Opus 5 1M) <noreply@anthropic.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EwxGB6TdoVvZ11KQQME9cL
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

include/akbasic/runtime.h

@@ -32,6 +32,7 @@
 #include <akbasic/structtype.h>
 #include <akbasic/symtab.h>
 #include <akbasic/types.h>
+#include <akbasic/ui.h>
 #include <akbasic/value.h>
 #include <akbasic/variable.h>
 
@@ -153,6 +154,13 @@ typedef struct akbasic_Runtime
     akbasic_AudioBackend *audio;
     akbasic_InputBackend *input;
     akbasic_SpriteBackend *sprites;
+    /*
+     * The fifth one, set on its own through akbasic_runtime_set_ui() rather than
+     * as a fifth argument to akbasic_runtime_set_devices(). Adding a parameter
+     * would have rewritten twenty-eight call sites to pass a NULL none of them
+     * cares about, in tests and documentation that are about something else.
+     */
+    akbasic_UiBackend *ui;
 
     /*
      * The graphics verbs' own state -- mode, color-source bindings, pixel cursor
@@ -172,6 +180,9 @@ typedef struct akbasic_Runtime
     /* GETKEY's hold on the step loop. Same reasoning again: it is the program's. */
     akbasic_InputState input_state;
 
+    /* Which menus are up, and GETMENU's hold on the step loop. */
+    akbasic_UiState ui_state;
+
     /*
      * The eight sprites, their positions and their collision bits. Same
      * reasoning as gfx and audio_state, and one more: RSPPOS and RSPRITE read
@@ -321,6 +332,21 @@ akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_init(akbasic_Runtime *obj, ak
  */
 akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_set_devices(akbasic_Runtime *obj, akbasic_GraphicsBackend *graphics, akbasic_AudioBackend *audio, akbasic_InputBackend *input, akbasic_SpriteBackend *sprites);
 
+/**
+ * @brief Attach the UI backend, where MENU, DIALOG, HUD and UISTYLE land.
+ *
+ * Separate from akbasic_runtime_set_devices() rather than a fifth argument to
+ * it, because that signature has twenty-eight call sites and none of them is
+ * about the UI. NULL withholds the capability, which is what the standalone
+ * stdio driver does and what every no-SDL build gets.
+ *
+ * @param obj Object to initialize, inspect, or modify.
+ * @param ui Where MENU, DIALOG, HUD and UISTYLE land; may be NULL.
+ * @return `NULL` on success, otherwise an error context owned by the caller.
+ * @throws AKERR_NULLPOINTER When `obj` is NULL.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_set_ui(akbasic_Runtime *obj, akbasic_UiBackend *ui);
+
 /**
  * @brief Tell the interpreter where the program it is running came from.
  *