Fix validation errors

The validator's messages share one grammar: a refdes, a colon, what is wrong, and — after the dash — what right would look like. "RV1: potentiometer needs an analog pin — GP15 is not an ADC pin (GP26/GP27/GP28)". Read it as three answers: WHO (RV1), WHAT (wrong pin kind), and WHICH PINS WOULD DO. Nothing to decode; the skill is believing it.

When a spec is broken several ways, fix in DEPENDENCY ORDER — the order the validator itself checks each peripheral, because later checks reason about facts the earlier ones establish. First the shape messages (a missing peripherals array stops everything). Then unknown parts — a peripheral the library cannot resolve reports once and is skipped, so its pins are invisible to later checks. Then required fields and signals, and a pins map that hands two signals one GPIO: "DS1: sevenseg_cc pins must be distinct (got GP2, GP2, GP4, GP5, GP6, GP7, GP8)". Then pin legality. Last, pin conflicts. One pass per fix, regenerate, read again.

That skipping is why fixes CASCADE. You met it in the PS-1 capstone: move the light sensor off GP15 to satisfy the analog check, and the conflict check — which could finally see a legal assignment — reports the pin was already spoken for. A new error after a fix is not the validator being fickle; it is the next layer of the same truth becoming visible. The board is done when a generate returns silence.

One failure class lives PAST the validator. Omit an LED's pin entirely and no named message fires — the generator itself fails, loudly and less politely, with tscircuit's own errors about components that could not connect. PS-1 Lesson 2 named the three classes: V7-checked, generator-failed, and optional. When you see machine noise instead of a sentence, look for what the spec never said at all.

Three seeded faults — find all three before reading on:

{
  "name": "Practice Board",
  "board": { "width": "100mm", "height": "80mm" },
  "mcu": { "part": "pico2w_socket", "refdes": "U1" },
  "peripherals": [
    { "part": "sevenseg", "refdes": "DS1",
      "pins": { "a": "GP2", "b": "GP3", "c": "GP4", "d": "GP5",
                "e": "GP6", "f": "GP7", "g": "GP8" },
      "series_resistor": "330ohm" },
    { "part": "potentiometer", "refdes": "RV1", "pin": "GP8",
      "value": "10k" },
    { "part": "pushbutton", "refdes": "SW1", "pin": "GP14",
      "pull": "internal_pullup" }
  ]
}