refactor: attach structured cause to v0 throws

[johnleider]

Summary

Every throw in packages/0/src now uses a typed V0Error class with a stable code discriminant, so consumers (and error trackers) can identify v0 errors without parsing message strings.

Site	Code
useContext — missing provider	V0_CONTEXT_MISSING (key)
useDate — plugin not installed	V0_PLUGIN_MISSING (plugin)
palettes/{ant,leonardo,material} — bad seed hex	V0_PALETTE_INVALID_SEED (palette, seed)
palettes/material — unknown variant	V0_PALETTE_UNKNOWN_VARIANT (palette, variant)
useLogger/adapters/{pino,consola} — missing instance	V0_ADAPTER_INSTANCE_MISSING (adapter)

import { isV0Error } from '@vuetify/v0'

try {
  useContext(myKey)
} catch (err) {
  if (isV0Error(err, 'V0_CONTEXT_MISSING')) {
    // err.code, err.key are typed
    reportToTracker(err.key)
  }
}

Design

Pattern modeled on tRPC's TRPCError and Node's error.code registry:

• V0Error extends Error with code: V0ErrorCode as a top-level field.
• Error.cause reserved for wrapping the genuine upstream error so Sentry's LinkedErrors integration renders the chain correctly. cause is forwarded through ErrorOptions and only set when supplied.
• V0ErrorDetails discriminated union (packages/0/src/types/index.ts) is the source of truth for what codes exist and what payload each carries. Adding a code = extending the union.
• isV0Error(err, code?) type guard intersects the instance with the matching V0ErrorDetails arm, so post-narrowing err.key / err.plugin / etc. are required (not optional).

Initial design used cause: { code, ... } as the discriminant payload. Review surfaced that this conflicts with the standard semantic of Error.cause — every precedent (Node, tRPC, Stripe SDK, OpenAI/Anthropic SDKs, Sentry's chain walker) uses cause for upstream wrapping, not as a discriminant. Pivoted before merging.

Public surface

Re-exported from @vuetify/v0:

• V0Error (class)
• isV0Error (type guard)
• V0ErrorCode (union of codes)
• V0ErrorDetails (discriminated union of constructor inputs)

Follow-ups (not in this PR)

• Lint rule (eslint-plugin-vuetify) — enforce that new throw statements under packages/0/src use V0Error, with documented carve-outs.
• Testing docs page — a dedicated guide covering mocks, fake timers, plugin testing, error assertions (including isV0Error narrowing), etc.

Verification

• pnpm typecheck:0 — clean
• pnpm build:0 — clean
• pnpm test:run packages/0 — 5862 passed / 18 skipped (added 2 cause-shape assertions to existing throw tests)
• grep "throw new Error" packages/0/src — 0 results outside test/bench files

[pkg-pr-new]

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commit: 27f8130