Feature: first-class multi-line text props — @multiline sugar + lint unknown @pattern values

[drubot]

Context

Drupal Canvas decides storability and translatability of a string prop structurally from its JSON schema shape. Multi-line plain text is recognized only as the exact literal shape { "type": "string", "pattern": "(.|\r?\n)*" } (Canvas PropShape::isPlainOrRichProse(), extended for translatability in canvas!1327, shipped in Canvas 1.8+). The pattern is a marker, not validation — it matches everything.

nuxt-component-preview currently passes @pattern through verbatim (detectPatternTag() in generateComponentIndex.ts) and documents it as "JSON Schema regex". That invites a silent failure mode: an author who uses @pattern for actual validation (e.g. ^[a-z-]+$) produces a prop shape Canvas cannot map to any field type — the prop becomes non-storable and non-translatable, with no feedback at authoring time and a hard-to-trace failure at Canvas registration.

Proposal

1. @multiline JSDoc tag as first-class sugar: emits exactly pattern: (.|\r?\n)* into the component index, so authors never type the magic literal. (Keep @pattern working for the literal for backwards compatibility.)
2. Lint at index generation: when @pattern carries any value other than the recognized literal, emit a build-time warning (or error behind a strict flag): "pattern X is not a Canvas-storable prop shape — the prop will be neither storable nor translatable; use @Multiline for multi-line text, an enum for constrained values, or runtime validation."
3. Docs: document the multi-line convention and the storable-shape constraint in the module docs / component-previews guide.

Why here

The authoring chain is JSDoc → nuxt-component-preview → component-index → Canvas discovery; this module is the only place that sees the author's intent early enough to warn before Drupal is involved.

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Filed via Claude Code (drunomics session; context: MCD-1149/Canvas 1.9 verification).