feat(feed-directory): last_result demotion and fail-closed catalog v2

[gildesmarais]

What changed

• Fail-closed Feed Directory client: accept catalog_version 2 only; require last_result on each catalog row (catalog-api.ts, types.ts).
• Demote empty/error rows in default sort; ambient last-scrape chrome in the browse UI (filters.ts, last-result.ts, render.ts, CSS).
• Warn before RSS subscribe, copy, and OPML export when the selection includes failing scrapes; honest filtered-count copy (no “ready-to-use” when empties/errors can be present) — assure fix in 7c94edc.
• Docs: describe instance last-known scrape hints vs /health (use-the-feed-directory.mdx, CONTEXT.md).

Why

Live browse depends on html2rss-web catalog API v2 exposing per-config last_result. Without fail-closed parsing and UI demotion, cold or failing scrapes look as healthy as successful ones, and bulk OPML/copy overstated readiness.

Risk

• Depends on html2rss-web shipping catalog API v2 for live browse; older API responses will fail closed (empty/error catalog load), not silently degrade.
• Ambient badges are last-known instance signals only — not live health.
• Low UI risk otherwise: demotion and warnings are additive; OPML still exports but warns.

Review map

1. src/components/feed-directory/adapters/catalog-api.ts — fail-closed v2 + required last_result
2. src/components/feed-directory/domain/last-result.ts + filters.ts — demotion / failing helpers
3. src/components/feed-directory/ui/render.ts + app/FeedDirectoryApp.ts — chrome, subscribe/copy/OPML warn paths
4. Matching *.test.ts — contract coverage for parse, sort, warn copy

Validation

• make check
• make build