audio: per-frame pitch envelopes for pulse SFX

Pulse-channel sfx with a multi-byte `pitch:` array used to silently
ignore everything past the first byte — the runtime audio tick
latched the period at trigger time and never updated it. Programs
that wanted a frequency sweep had no way to express it.

The compiler now compiles a per-frame pitch envelope blob alongside
the existing volume envelope when `decl.pitch` has more than one
distinct value. The blob is padded (or truncated) to the volume
envelope's length and ends in a zero sentinel so the runtime
walker stops both pointers on the same NMI. Sfx with a single
scalar pitch (or an array where every byte is the same) keep their
historical "no pitch blob, latch once" path and emit byte-identical
ROM bytes.

The runtime gains two new pieces, both gated on a new
`__sfx_pitch_used` codegen marker so programs without varying-pitch
sfx pay zero bytes:

1. `gen_audio_tick` emits a per-frame pitch update block inside
   the SFX tick: read a byte through `(AUDIO_SFX_PITCH_PTR),Y`,
   write it to `$4002` (pulse-1 period low), advance the pointer.
   The block bails on a zero high-byte pointer so a single
   program can mix scalar-pitch and varying-pitch sfx without
   one clobbering the other.

2. `emit_play_pulse` seeds `AUDIO_SFX_PITCH_PTR_LO/HI` with the
   pitch-blob label for varying-pitch sfx and zeros it for
   scalar-pitch sfx. The per-call branch is skipped entirely
   when the program has no varying-pitch sfx anywhere.

The new `examples/sfx_pitch_envelope.ne` exercises the path with
a 16-frame siren sweep. Triangle and noise per-frame pitch are
deferred — they share the same data shape but the runtime ticks
for those channels still write only their volume registers, see
docs/future-work.md for the gap.

https://claude.ai/code/session_01KEczoNUX3WmcFLfq6iAQxB

examples/sfx_pitch_envelope.ne

@@ -0,0 +1,42 @@
+// Per-frame pitch envelope on a pulse SFX. The user authors the
+// `pitch:` array as one byte per frame (matching the per-frame
+// `volume:` array) and the runtime audio tick walks both pointers
+// in lockstep, writing pitch to `$4002` and volume to `$4000` on
+// every NMI. The result is a frequency-sweeping "siren" tone — the
+// classic latch-once pulse SFX driver couldn't model this at all.
+//
+// Per-frame pitch is opt-in: if the `pitch:` array has a single
+// value (or repeats one byte) the compiler emits the byte-identical
+// pre-pitch-envelope sequence and no extra blob, so existing
+// programs that just want a static pitch keep working unchanged.
+// See `runtime/gen_audio_tick` for the gated extension.
+//
+// Build: cargo run -- build examples/sfx_pitch_envelope.ne
+
+game "SFX Pitch Envelope" {
+    mapper: NROM
+}
+
+// 16-frame pitch sweep from $40 down to $20 paired with a slow
+// volume ramp. Both arrays are the same length so the runtime's
+// lockstep walker handles the simplest possible case.
+sfx Siren {
+    duty: 2
+    pitch:  [0x40, 0x3D, 0x3A, 0x37, 0x34, 0x31, 0x2E, 0x2B, 0x28, 0x26, 0x24, 0x22, 0x21, 0x20, 0x20, 0x20]
+    volume: [15,   14,   13,   12,   11,   10,   9,    8,    7,    6,    5,    4,    3,    2,    1,    0]
+}
+
+var tick: u8 = 0
+
+on frame {
+    // Re-trigger the sfx every 60 frames so the siren restarts
+    // whenever the previous pass mutes itself, giving the
+    // emulator harness a stable pattern to capture.
+    tick += 1
+    if tick == 60 {
+        tick = 0
+        play Siren
+    }
+}
+
+start Main