linker: test that BackgroundData::chr_bytes lands at chr_base_tile

Adds a focused unit test that constructs a `SpriteData` at tile 1
and a `BackgroundData` whose `chr_bytes` claim tiles 5-6, then
verifies the linker's CHR ROM placement preserves the smiley at
tile 0, the sprite at tile 1, leaves tiles 2-4 untouched, and
copies the background blob at the requested base offset. Catches
any future regression in the
`BackgroundData::chr_base_tile` → CHR ROM splice that
`assets: auto-generate CHR data from @nametable() PNG sources`
introduced.

https://claude.ai/code/session_01KEczoNUX3WmcFLfq6iAQxB

src/linker/tests.rs

@@ -130,6 +130,56 @@ fn link_with_sprites_spanning_multiple_tiles() {
     assert_eq!(placed, sprite_bytes.as_slice());
 }
 
+#[test]
+fn link_with_background_chr_places_blob_after_sprites() {
+    // A `BackgroundData` carrying its own CHR data should drop
+    // the bytes into CHR ROM at `chr_base_tile * 16`. The
+    // linker must NOT touch any earlier tiles (the smiley at
+    // tile 0 plus any sprites). We seed a sprite at tile 1 and
+    // a background at tile 5, then verify the linker placed
+    // both blobs at their expected offsets.
+    let linker = Linker::new(Mirroring::Horizontal);
+    let user_code = vec![Instruction::implied(NOP)];
+    let sprite_bytes: Vec<u8> = vec![0xAA; 16]; // tile 1
+    let bg_chr: Vec<u8> = (0..32).map(|i| i as u8).collect(); // tiles 5, 6
+    let sprites = vec![SpriteData {
+        name: "Player".into(),
+        tile_index: 1,
+        chr_bytes: sprite_bytes.clone(),
+    }];
+    let backgrounds = vec![crate::assets::BackgroundData {
+        name: "Stage".into(),
+        tiles: [5u8; 960],
+        attrs: [0u8; 64],
+        chr_bytes: bg_chr.clone(),
+        chr_base_tile: 5,
+    }];
+    let rom = linker.link_banked_with_ppu(&user_code, &sprites, &[], &[], &[], &backgrounds, &[]);
+    let chr_start = 16 + 16384;
+    // Sprite bytes still at tile 1.
+    assert_eq!(
+        &rom[chr_start + 16..chr_start + 32],
+        sprite_bytes.as_slice(),
+        "sprite tile bytes should survive the background CHR splice"
+    );
+    // Background CHR at tile 5 (offset 80).
+    assert_eq!(
+        &rom[chr_start + 80..chr_start + 80 + 32],
+        bg_chr.as_slice(),
+        "background CHR bytes should land at chr_base_tile * 16"
+    );
+    // Tiles 2/3/4 should still be all zeros — the linker mustn't
+    // shadow the gap between the sprite tile and the background.
+    for tile in 2..5usize {
+        let off = chr_start + tile * 16;
+        assert_eq!(
+            &rom[off..off + 16],
+            &[0u8; 16],
+            "tile {tile} should remain zero between sprite and background"
+        );
+    }
+}
+
 #[test]
 fn link_splices_audio_tick_when_user_marker_present() {
     // When user code contains the `__audio_used` marker label (the