feat(pio-peripheral): let a peripheral take the words instead of the PIO

Two additions, both for peripherals that model the far end of the wire
rather than watch it arrive bit by bit.

feedWord() now carries which state machine wrote the word, where the host
plumbing knows it, so a peripheral can tell one program from another.

consumesTxWords() is the new one that matters: it says "I have this word,
the PIO does not need to shift it out". A Pimoroni Unicorn's panel spends
about 300,000 PIO cycles per refresh in the program's BCD delay loops, and
every one of them is wasted once the pixels have been read out of the FIFO
word. Unlike inDiscardableWriteData(), the peripheral is still fed the
word — that is the whole point of it.

frontend/src/simulation/PioPeripheral.ts

@@ -21,11 +21,37 @@
 
 export interface PioPeripheral {
   /** Process one 32-bit word the firmware bit-banged onto the bus. Returns
-   *  zero or more reply byte-blobs to repack into the RX FIFO. */
-  feedWord(word: number): Uint8Array[];
+   *  zero or more reply byte-blobs to repack into the RX FIFO.
+   *
+   *  `pioIndex`/`smIndex` say which state machine wrote it, where the host
+   *  plumbing knows (the RP2350 hook passes them; the RP2040 one does not, so
+   *  a peripheral that needs them must cope with undefined). A single-protocol
+   *  peripheral can ignore both. */
+  feedWord(word: number, pioIndex?: number, smIndex?: number): Uint8Array[];
   /** True while the firmware is streaming bulk data the peripheral wants the
-   *  plumbing to DISCARD (keep only a few words so the PIO TXSTALLs). */
+   *  plumbing to DISCARD (keep only a few words so the PIO TXSTALLs). Words
+   *  taken by this path are NOT fed to the peripheral. */
   inDiscardableWriteData(): boolean;
+  /**
+   * Optional: "I have already consumed this state machine's words; the PIO
+   * does not need to shift them out."
+   *
+   * For a peripheral that models the far end of the wire directly — an LED
+   * panel whose framebuffer we decode rather than watch arrive bit by bit —
+   * running the PIO program is pure cost. A Pimoroni Unicorn spends about
+   * 300,000 PIO cycles per refresh sitting in the program's BCD delay loops,
+   * which buys nothing once the pixels have been read out of the FIFO word.
+   *
+   * When this returns true the plumbing feeds the peripheral as usual, then
+   * drops the word instead of handing it to the PIO, and keeps TX DREQ
+   * asserted so a DREQ-paced DMA still advances. Unlike
+   * inDiscardableWriteData(), the peripheral still SEES every word — that is
+   * the whole point.
+   *
+   * Called per state machine so a peripheral only claims the one it has
+   * locked onto; another program on another SM keeps working normally.
+   */
+  consumesTxWords?(pioIndex: number, smIndex: number): boolean;
   /** Reset framing at a transfer boundary (the PIO sm.restart). */
   resetFraming(): void;
   /** Current host-wake level to drive onto GPIO24 (active-high). */