ECCA: extend catalog-builder + catalog/index.html to render event products

[enricopiovesan]

Summary

Split off #154's second DoD bullet. Once the registry-side query surface
(#154, done) exists, extend capability-src/catalog-builder/ (WASM, emits
catalog.json), scripts/ci/gather_catalog_data.py, and
catalog/index.html to actually render event products in the static
discovery catalog.

Why blocked (updated)

Originally framed as "waiting on real event-product data." Turns out
that's understated: there's no pipeline yet for that data to exist as
real, governed content at all -- tracked now as its own initiative in
#168. capabilities/ has no events/ sibling, capability_validation.py
has zero event-contract validation, build_index.py has zero event
awareness. This issue is blocked on #168, not just on "someone publishes
an event" -- #168 has to exist first.

Governing spec

specs/016-ecca-event-product-adoption -- FR-010 (the UI-rendering
half; the query-surface half, #154, is done).

Definition of Done

• capability-src/catalog-builder/ extended to emit event-product data
  into catalog.json.
• catalog/index.html renders event products with filters by event,
  capability, domain, owner, lifecycle, and classification.
• Verified against at least one real published event product, not only
  synthetic fixtures.

Depends on

#168 (build the publishing pipeline first -- no real data can exist without it).

[enricopiovesan]

Board-hygiene fix (registry-ops backlog-gardener pass): corrected Project 3 status from Ready to Blocked, matching this issue's own body ("blocked on #168, not just on 'someone publishes an event'"). #168 itself is deferred pending real demand, not actively being worked, so this stays blocked behind it.