[Xuepoo]

What this adds

New candidate page extensibility/ui-component-catalog-candidate.md, indexed in
extensibility/README.md: five named composition recipes over the accepted
Plugin API v1 declarative surface, plus the bar plugin mapping.

ID	Pattern	Surface status
C-1	Edge band	Accepted v1
C-2	Pill row	Accepted v1 (no radius, border, or padding field)
C-3	Tooltip / detail fallback	Partially available
C-4	Modal sheet	Not available in plugin API v1
C-5	Status strip	Accepted v1

Each entry states its surface status first, then scene shape, style hooks,
interaction, accessibility notes, budget note, and a headed Lua example.

Availability is stated, not assumed

• Accepted v1. C-1, C-2, and C-5 use only Text/Row/Column/List,
  fg/bg/bold, and on_click/on_scroll with region.
• Partially available. C-3 is a four-rung ladder: inline text and
  bitty.notify are accepted; the presentation-only overlay (ui.present) is
  current host fact and is explicitly not in the accepted RFC; anchored tooltips
  do not exist.
• Not available. C-4 has no scene shape. Plugin API v1 carries no modal
  authority, ui.present admits only a non-modal surface, and
  register_panel/PanelProvider stay excluded from v1. The page documents the
  substitutes and their limits instead of sketching a sheet.

No entry adds a node kind, slot, field, capability, budget, or error code, and no
candidate direction is presented as a contract.

Bar mapping

The bar plugin mapping records which pinned bar element matches which recipe
(band root row, capsules, marks, detail notification fallback, per-module
omission). It states explicitly that the bar does not import these recipes as
code, so no dogfood proof of shared implementation is claimed.

Verification

just check cannot complete in the authoring environment: bun/bunx
segfault (signal 11) on every invocation, so the fmt-check recipe dies with
exit 139 before the remaining gates run. Every gate was therefore run through the
pinned equivalents, all green:

• prettier@3.9.6 --check . --ignore-unknown (0 issues)
• markdownlint-cli2@0.23.1 (0 issues, 106 files)
• check-docs.mjs links|metadata|language|agents|hygiene (each 0)
• xmllint svg gate and actionlint 1.7.12 (0)

Issue

Related #2114, not Closes. Checklist item 1 (catalog the band, pill row,
tooltip/detail fallback, modal sheet, and status strip with contracts) and item 2
(scene shape, style hooks, accessibility notes, and a headed example per entry)
are covered by this page. Item 3, "Bar migrated onto catalog entries (dogfood
proof)", is not: this PR maps the bar's pinned behaviour but does not migrate the
bar plugin, which lives in its own repository.