audio: triangle and noise sfx channels

Adds `channel: triangle` / `channel: noise` to the `sfx` declaration
form. The existing pulse-1 / pulse-2 driver is unchanged (and is
still byte-identical for programs that don't use the new channels)
— when a program declares a triangle or noise sfx the runtime
splices in an additional per-channel slot that writes to $4008-
$400B (triangle) or $400C-$400F (noise) on play. Includes a new
`examples/noise_triangle_sfx.ne` demo with committed golden PNG +
audio hash.

https://claude.ai/code/session_01MaNVcDmK9gsspRkdxowQAM

examples/README.md

@@ -28,6 +28,7 @@ Open any `.nes` file in an NES emulator ([Mesen](https://www.mesen.ca/), [FCEUX]
 | `mmc1_banked.ne` | MMC1, banks, multiply | Banked mapper with software multiply |
 | `palette_and_background.ne` | palette, background, set_palette, load_background | Reset-time initial load plus vblank-safe runtime swaps |
 | `friendly_assets.ne` | named colours, grouped palette, pixel art, tilemap+legend, palette_map, scalar sfx pitch, note-name music | Exercises every "friendlier" asset syntax at once — the `palette` uses `bg0..sp3` + a shared `universal:`, the sprite is authored as ASCII pixel art, the background uses a `legend { ... } + map:` tilemap with a `palette_map:` for attributes, the sfx uses a scalar `pitch:` + `envelope:` alias, and the music uses note names (`C4, E4 40, rest 10`) with a `tempo:` default. |
+| `noise_triangle_sfx.ne` | `channel: noise`, `channel: triangle` on `sfx` blocks | Demonstrates the noise and triangle sfx channels. Declares one noise burst and one triangle bass note, plays each on a timer so the emulator harness captures both the pixel output and the APU state. |
 | `platformer.ne` | **every subsystem** | End-to-end side-scrolling demo: custom CHR tileset, full 32×30 nametable with per-region attribute palettes, 2×2 metasprite hero with gravity/jump physics, wrap-around horizontal scrolling, stomp-or-die enemy collisions with a live stomp-count HUD, coin pickups, user-declared SFX + music, and a Title → Playing → GameOver state machine with a proximity-based autopilot so the headless harness cycles through stomp, stomp, die, and retry inside six seconds. Regenerate the tile art with `cargo run --bin gen_platformer_tiles`. |
 
 ## Emulator Controls

examples/noise_triangle_sfx.ne

@@ -0,0 +1,70 @@
+// Noise / Triangle SFX Demo
+//
+// Showcases the newer `channel:` property on `sfx` blocks. The audio
+// driver's per-frame tick gains a noise channel (writes to $400C) and
+// a triangle channel (writes to $4008) whenever the program declares
+// at least one sfx targeting those channels. Programs that stick to
+// pulse 1 still emit the exact same driver code as before.
+//
+// Every 30 frames we trigger one of the two effects and move a small
+// smiley back and forth so it's obvious the program is still running.
+// The emulator harness sees the registers get poked and hashes the
+// APU output, so the golden locks in both the pixels *and* the sound.
+//
+// Build: cargo run -- build examples/noise_triangle_sfx.ne
+// Output: examples/noise_triangle_sfx.nes
+
+game "Noise Triangle SFX" {
+    mapper: NROM
+}
+
+// A short, sharp noise burst — perfect for explosions / footsteps.
+// `pitch: 4` indexes the APU's 16-entry noise period table; lower
+// values are higher-pitched. `volume` is a per-frame amplitude ramp,
+// exactly like a pulse sfx.
+sfx Crash {
+    channel: noise
+    pitch: 4
+    volume: [15, 13, 11, 9, 7, 5, 3, 1]
+}
+
+// A sustained triangle "bass" note. Triangle has no volume register,
+// so `volume:` entries are just "hold" flags — nonzero means sustain,
+// zero means silence. The numeric value doesn't matter.
+// `pitch: 60` picks a period-table entry; triangle shares the pulse
+// period table, and 60 is the lowest note in it (C1).
+sfx Bass {
+    channel: triangle
+    pitch: 60
+    volume: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
+}
+
+var px: u8 = 120
+var timer: u8 = 0
+var bounce: u8 = 0
+
+on frame {
+    // Bounce the smiley between x=110 and x=140 so frame 180 is
+    // not a still frame — the golden image diff would otherwise
+    // miss the "program is running" signal.
+    timer += 1
+    if bounce == 0 {
+        px += 1
+        if px == 140 { bounce = 1 }
+    } else {
+        px -= 1
+        if px == 110 { bounce = 0 }
+    }
+
+    // Cycle through the two sfx every 30 frames so each channel
+    // retriggers at least twice inside the 180-frame capture window.
+    if timer == 30 { play Crash }
+    if timer == 60 {
+        timer = 0
+        play Bass
+    }
+
+    draw Smiley at: (px, 120)
+}
+
+start Main