docs(widgets): complete the v0 element pages, first Surface pages, and named patterns

[k08200]

Signalled in #22. Docs only — 10 files, +1,599 lines, no code changes. Every v0 page declares Lifetime: Disposable with the v0 widget draft per the docs constitution; stable hubs stay version-neutral.

What this adds

Element pages (5 new → catalog coverage 10/10): textfield, region, text, pager, video — TEMPLATE.md meta blocks, implemented-vs-proposed labelling per claim, grounded in catalog/checker/core/renderer line references, real corpus usage, honest gap lists.

First Surface pages (2): sheet (implemented default modality: structural reserved dismiss via Escape/scrim, focus-restoring dismiss descriptor, occlusion) and dialog (honestly labelled: named by a checker steering note only — no design, no realization, no corpus usage; every surface currently renders as a bottom sheet).

Named patterns (2): avatar and card under the stable patterns/ hub — the two names the checker's unknown-element notes already direct authors to; version-neutral, deferring exact contracts to the versioned pages. The surfaces hub's "none documented yet" line now routes to the v0 pages.

Findings surfaced while grounding claims (candidate redesign evidence)

• modality is completely unvalidated — the parser accepts any identifier; no checker rule exists.
• pager's page-change is declared in the catalog but wired in no renderer (zero references in web/src).
• The video "deferred" note in UNKNOWN_ELEMENT_NOTES (markup.rs:85-88) is unreachable dead data — catalog names classify as CatalogElement before the notes are consulted. Catalog (base.toml:79-107) is current truth; adjudication asked in docs(widgets): contribute v0 draft pages for the 5 undocumented elements and the first Surface pages #22.
• The §10 cited by the catalog's controlled-promotion comment resolves to the Instagram spike design's §10, not the v0 spec draft's §10.
• textfield has no uncontrolled draft semantics (an unbound field's typing is clobbered by reconciliation), aria-label-only naming, and no dedicated unit test for the Play in-flight/stash/IME mechanics.
• Region's UH5020 (supplementary reachability) is enforced in code but has no golden rejection.

Draft on purpose: happy to hold, split, or reshape per the redesign timeline.

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[k08200]

Closing: this PR's target (the v0 widget draft line) is retired by #24. The 10 pages remain on the branch (docs/widget-textfield) as an accurate record of the 0.3 contracts and their measured gaps — reopenable as a 0.3 historical record if wanted, and the gap findings carry forward via the question on #24.