Fix HUD vertical flip: dpl2d Y grows DOWN, not up

User-reported live: the torso-twist tape appeared at the TOP of the reticle
(should be bottom) and the range ladder filled the wrong direction. I had
assumed dpl2d used a standard math y-up convention (y0=bottom, y1=top) and
inverted accordingly.

Found the exact, already-solved answer in vrview_gl.py's own HUD vertex
shader comment: "dpl2d Y grows DOWN (screen convention): period VHS footage
shows the twist dial UNDER the reticle and the range bar filling top-to-
bottom; mapping straight onto GL's Y-up NDC rendered the whole HUD flipped"
-- the exact same bug, already diagnosed and fixed there. y0 (the smaller
raw view-rect value) is the TOP of screen, y1 the bottom -- direct mapping,
no inversion needed.

Verified offline: the composited reticle now shows the twist tape (with its
bowtie shape) at the bottom and the range ladder/pips reading top-to-bottom,
matching BT411's documented layout.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

emulator/firmware-decomp/emu860c/hud2d_overlay.py

@@ -28,7 +28,8 @@ _EDGE_OP = {6: 'nest', 7: 'link', 8: 'prune', 11: 'list_add', 12: 'list_remove'}
 
 class HudTracker:
     """Incremental wire-record feed -> the view's live dpl2d overlay
-    primitives, in view-rect space (x0..x1, y0..y1, y up).
+    primitives, in view-rect space (x0..x1, y0..y1, y grows DOWN --
+    y0 is the top of screen, y1 the bottom; see composite_hud's to_px).
 
     Only tracks what hud2d_root/hud2d_prims need: node type+body (create/
     flush), the dcs->view list_add edge, and the dl2d display-list words
@@ -101,8 +102,9 @@ class HudTracker:
 
 def composite_hud(img, tracker, W, H):
     """Draw the tracker's current HUD primitives onto img (HxWx3 uint8,
-    modified in place). view-rect (x0..x1, y0..y1, y UP) -> pixel space
-    (0..W, 0..H, y DOWN): px = (x-x0)/(x1-x0)*W, py = (1-(y-y0)/(y1-y0))*H.
+    modified in place). view-rect (x0..x1, y0..y1, y grows DOWN -- confirmed
+    live, see to_px) -> pixel space (0..W, 0..H, y down, same direction):
+    px = (x-x0)/(x1-x0)*W, py = (y-y0)/(y1-y0)*H, no inversion.
     No-op (returns False) if the view rect or display list isn't resolved
     yet -- callers should keep showing the last good frame, same as the 3D
     render's own "skip if not ready" convention."""
@@ -121,7 +123,13 @@ def composite_hud(img, tracker, W, H):
 
     def to_px(pt):
         x, y = pt
-        return ((x - x0) / dx * W, (1.0 - (y - y0) / dy) * H)
+        # dpl2d Y grows DOWN (screen convention), confirmed both by the user
+        # (live: horizontal/twist tape appeared at top instead of bottom, the
+        # range ladder filled the wrong direction) and by vrview_gl.py's own
+        # comment/fix for the identical symptom ("mapping straight onto GL's
+        # Y-up NDC rendered the whole HUD flipped") -- y0 (the smaller raw
+        # value) is the TOP of screen, y1 the bottom. No inversion needed.
+        return ((x - x0) / dx * W, (y - y0) / dy * H)
 
     for kind, col, width, pts in prims:
         if len(pts) < 1: