platformer: move HUD from sprite OAM slots to background + sprite-0 split

The status bar now paints into NT row 1 (coin + score digits on
the left, heart + lives digit on the right) using the `bg3`
sub-palette that matches `sp0` pixel-for-pixel. A single OAM
slot-0 anchor sprite sits over the coin tile; its one opaque
pixel lines up with the coin's bottom row so sprite-0 hit fires
at scanline 15, and a trailing `sprite_0_split(camera_x, 0)`
latches the playfield scroll starting at scanline 16. NT rows
0-1 stay pinned while scanlines 16+ scroll with the camera.

Score / lives updates are shadow-compared (`last_score`,
`last_lives`) so the VRAM ring sees an entry only when the
backing state actually changes — most frames append zero bytes.
OAM footprint drops from 5 sprites per frame down to 1.

Tile pipeline gains a 27th entry — a 7-transparent-row + 1-pixel
anchor — so the sprite-0 hit lands on scanline 15 instead of
scanline 8 (the latter would smear the HUD glyphs across the
split). `gen_platformer_tiles.rs` is updated in lockstep.

Ancillary changes: `bg3` retuned from `[yellow, orange,
dk_orange]` to `[red, orange, white]` (matching `sp0`);
`palette_map` row 0 flips from bg0 to bg3; legend gains `o`, `h`,
`0`, `3` so the initial map can preload the static HUD tiles and
the committed nametable already reads "coin 00 ... heart 3" on
frame 0.

`docs/future-work.md` loses the sprite-0 HUD follow-up section
(this commit lands it). Goldens + gif refreshed.

scripts/gen_platformer_tiles.rs

@@ -367,6 +367,30 @@ aaaaaaaa
 ...aa...
 ........",
     },
+    // Sprite-0 hit anchor — a single opaque pixel at row 7, col 3,
+    // everything else transparent. Sits behind the HUD row's coin
+    // tile as OAM slot 0 in every `on frame`; its one opaque pixel
+    // aligns with column 3 of the coin's row 7 (`...bb...`), so
+    // the PPU sets the sprite-0 hit flag at scanline 15 — the
+    // last scanline of the HUD row. Writing `$2005` the moment
+    // that flag sets means the horizontal scroll flip takes
+    // effect at scanline 16 (the PPU latches horizontal scroll at
+    // the next HBLANK), which pins NT rows 0-1 at scroll=0 and
+    // lets scanlines 16+ render at `camera_x`. A sprite that
+    // hit on its *top* row (e.g. using the coin as sprite 0)
+    // would flip the scroll mid-HUD-row and smear the glyphs.
+    Tile {
+        name: "Sprite 0 anchor",
+        art: "\
+........
+........
+........
+........
+........
+........
+........
+...c....",
+    },
 ];
 
 // ── Named CHR tile indices used by the nametable layout below ──