runtime: gate OAM DMA and OAM shadow init on __oam_used

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Skip the OAM DMA (LDA#0/STA \$2003 + LDA#2/STA \$4014) inside the
NMI handler and the `\$FE` hide-sentinel fill of the \$0200 OAM
shadow inside `gen_init` for programs that never `draw`. Both are
gated on the `__oam_used` marker the IR codegen now drops at the
first `IrOp::DrawSprite`.

Savings per NMI for a non-drawing program:
 - ~520 cycles (the DMA is 513 cycles plus the 4 register writes),
 - ~9 bytes of NMI code,
 - ~4 bytes of init code (the \$FE swap is replaced by a plain
   zero-fill of \$0200-\$02FF alongside the rest of the 2 KB RAM
   clear).

Plumbed by:
 - New `NmiOptions::has_oam: bool`, threaded through `gen_nmi`.
 - `gen_init(has_oam: bool)` parameter controlling the inner-loop
   OAM fill. Existing runtime tests all migrate to `gen_init(true)`
   to preserve their legacy assertions.
 - Linker computes `has_oam = has_label(user_code, "__oam_used")`
   once and feeds it to both call sites, and the existing
   `has_visual_output` predicate reuses the same lookup rather than
   re-scanning user_code.

sfx_pitch_envelope is the one audio-only example; its audio
golden flips by the usual cycle-accurate-APU-register-write-timing
drift caused by the NMI layout shifting ~14 bytes earlier.

https://claude.ai/code/session_016kM6P7PukktBDqTZexrrAN

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Claude 2026-04-16 13:38:27 +00:00

parent 6561daff35

commit bd30ac3010

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examples/auto_chr_background.nes

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examples/sfx_pitch_envelope.nes

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