runtime: gate controller-2 reads in NMI on __p2_input_used

Drop the three-instruction JOY2 shift block (`LDA $4017 / LSR A /
ROL ZP_INPUT_P2`) from inside the NMI's 8-iteration input loop
when user code never reads controller 2. IR codegen emits the
`__p2_input_used` marker from `IrOp::ReadInput(_, 1)`; the linker
threads the flag through a new `NmiOptions::has_p2_input` bool,
and `gen_nmi` writes the shift block only when the flag is set.

Savings for single-player programs:
 - ~6 bytes of NMI code.
 - ~30 cycles per frame (3 instructions × 8 loop iterations, each
   6-8 cycles depending on addressing — LDA abs is 4, LSR A is 2,
   ROL zp is 5, so ~11 cycles × 8 = ~88 cycles; rounded down for
   the page-crossing penalty landing differently in the new layout).

This commit also fixes the IR codegen to drop the matching
`__p1_input_used` marker from `IrOp::ReadInput(_, 0)`, even though
the next commit is the one that actually consumes it. Landing the
two markers together keeps the IR codegen's per-op bookkeeping
coherent.

Six audio goldens flip (every program that reads input + plays
audio) with the expected NMI-layout-shift cycle drift.

https://claude.ai/code/session_016kM6P7PukktBDqTZexrrAN

tests/emulator/goldens/audio_demo.audio.hash

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tests/emulator/goldens/friendly_assets.audio.hash

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tests/emulator/goldens/noise_triangle_sfx.audio.hash

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tests/emulator/goldens/platformer.audio.hash

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tests/emulator/goldens/sfx_pitch_envelope.audio.hash

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tests/emulator/goldens/war.audio.hash

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