spec(t27f): demo camera and scene kind as scalar entry points for deck hosts

specs/t27f/reel.t27

@@ -24,7 +24,7 @@
 ; answer reaches the user everywhere; the desktop Code tab opens it in its Browser pane. CHANNEL_* below.
 ; Host: gHashTag/t27-mcp t27f_view draws stills of the demo reel from a wasm build of this file's
 ; Zig output (bodies) and its gen-js output (constants); see "host entry points" below.
-; Not claimed: no video has been rendered, and no host but t27f_view imports this file.
+; Not claimed: no video has been rendered. Importers: t27f_view (above) and T27fDeck (specs/t27f/deck.t27).
 ; phi^2 + 1/phi^2 = 3 | TRINITY
 
 module T27fReel;
@@ -514,6 +514,36 @@ pub fn demo_r(f: i32, preset: u8) -> i32 {
     return screen_r(camera_hw(DEMO_COUNT, DEMO_KIND, DEMO_CELL, DEMO_FRAMES, f), preset);
 }
 
+/// The demo camera itself at base frame f, for a host that places the map somewhere other than the
+/// full frame (T27fDeck slide_x / slide_y / slide_r take exactly these three).
+pub fn demo_cx(f: i32) -> i32 {
+    return camera_cx(DEMO_COUNT, DEMO_KIND, DEMO_CELL, DEMO_FRAMES, f);
+}
+
+pub fn demo_cy(f: i32) -> i32 {
+    return camera_cy(DEMO_COUNT, DEMO_KIND, DEMO_CELL, DEMO_LENS, DEMO_FRAMES, f);
+}
+
+pub fn demo_hw(f: i32) -> i32 {
+    return camera_hw(DEMO_COUNT, DEMO_KIND, DEMO_CELL, DEMO_FRAMES, f);
+}
+
+/// The kind of a demo scene, or SCENE_KIND_COUNT (no kind) for a scene the demo does not have.
+pub fn demo_kind(scene: i32) -> u8 {
+    if (scene < 0 or scene >= DEMO_COUNT) {
+        return SCENE_KIND_COUNT;
+    }
+    return DEMO_KIND[@intCast(scene)];
+}
+
+/// The first base frame of a demo scene, or END_OF_REEL.
+pub fn demo_scene_start(scene: i32) -> i32 {
+    if (scene < 0 or scene >= DEMO_COUNT) {
+        return END_OF_REEL;
+    }
+    return scene_start(DEMO_FRAMES, scene);
+}
+
 /// Corner i of a cell on screen: what a host draws as the cell's polygon.
 pub fn demo_corner_sx(f: i32, preset: u8, cell: i32, i: i32) -> i32 {
     return demo_sx(f, preset, cell_x(cell) + corner_dx(i));
@@ -750,6 +780,32 @@ test demo_entry_points_match_the_reel {
     assert demo_sy(demo_still(1), PRESET_WIDE, lens_cy(0)) == 360;
 }
 
+test demo_camera_is_the_reel_camera {
+    // the camera a host reads is the one demo_sx / demo_sy / demo_r already use
+    assert demo_cx(224) == cell_x(10);
+    assert demo_cy(224) == cell_y(10);
+    assert demo_hw(224) == FOCUS_HW;
+    assert demo_cy(demo_still(1)) == lens_cy(0);
+    var s : i32 = 0;
+    while (s < DEMO_COUNT) {
+        const f : i32 = demo_still(s);
+        assert demo_sx(f, PRESET_TALL, 0) == screen_x(demo_cx(f), demo_hw(f), PRESET_TALL, 0);
+        assert demo_sy(f, PRESET_TALL, 0) == screen_y(demo_cy(f), demo_hw(f), PRESET_TALL, 0);
+        assert demo_scene_at(demo_scene_start(s)) == s;
+        assert demo_scene_start(s) <= f;
+        s = s + 1;
+    }
+    // the demo has no title scene: scene 0 is the overview, scene 4 the call
+    assert demo_kind(0) == SCENE_OVERVIEW;
+    assert demo_kind(2) == SCENE_FOCUS;
+    assert demo_kind(4) == SCENE_CALL;
+    assert demo_kind(5) == SCENE_KIND_COUNT;
+    assert demo_kind(-1) == SCENE_KIND_COUNT;
+    assert demo_scene_start(0) == 0;
+    assert demo_scene_start(2) == 135;
+    assert demo_scene_start(5) == END_OF_REEL;
+}
+
 test overview_still_fits_every_frame {
     // every corner of every cell of the first still lands inside the picture on every preset
     var p : u8 = 0;