linker: gate default smiley CHR tile on __default_sprite_used marker

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Drop the built-in smiley from CHR tile 0 unless something in the
program actually references it. The marker fires when either:

  1. `IrOp::DrawSprite` lowering falls back to tile 0 because the
     sprite name doesn't resolve to a user declaration, or
  2. The same lowering sees a runtime `frame:` override (which
     could index any tile, including 0).

A third source of dependency — a background nametable entry of 0 —
is detected in the linker by scanning `bg.tiles` for zeros. This
preserves the smiley for programs like `examples/friendly_assets`
that use tile 0 as a background placeholder, even though their
draws resolve to user-declared sprites.

Programs whose draws all resolve to explicitly-declared sprites
with static frames AND whose backgrounds reference tiles 1+ now
leave CHR tile 0 as an all-zero blank, freeing 16 CHR bytes that
the user can treat as an always-transparent background tile.
Verified against the current example set: `sprites_and_palettes`
and `auto_chr_background` reclaim tile 0; every other example
keeps it (either they fall back to tile 0 via an undeclared draw
name or their background tilemap references tile 0).

All 33 emulator goldens still pass — removing an unreferenced CHR
tile can't change observable output.

https://claude.ai/code/session_016kM6P7PukktBDqTZexrrAN

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Claude 2026-04-16 13:32:52 +00:00

parent 7533ac281e

commit 6561daff35

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