fix(web): address the second review round on context provenance

- An empty replacement catalog is a real catalog: `renderCatalogUpdate()`
  publishes zero entries when the last skill disappears, and falling back
  would hide that every earlier name was retired.
- The opaque fallback keeps a `form` declaration this version cannot present.
  It is the one place a newer or foreign log's declared shape would otherwise
  vanish from the UI entirely, since the row marker is also absent there.
- An instruction change with an unrecognized `action` disqualifies the record.
  The action decides the word the row shows, so an unknown one would be
  presented as loaded or updated.
- The catalog list bounds itself and reports the withheld count. Entry count
  is unbounded and the scrollport bounds height, not node count.
- A catalog message keeps content blocks this version does not know, instead
  of dropping model-visible content the extensible union may carry.
- `core.md` defines `ContextFormed`, the interface actually carrying the
  optional field, beside `ContextForm`.
- The superseded-in-part bullet states the affected clauses as one rule rather
  than enumerating them; two rounds of enumeration each missed some, which is
  the shape being fragile rather than the list being wrong.
- The note records the one migration case that does not self-heal: an
  old-format catalog as the only one, with an empty current view, leaves a
  stale catalog nothing replaces.

docs/core-data-structures/core.md

@@ -193,6 +193,13 @@ type ContextForm =
   | 'catalog'
 ```
 
+```ts type-equiv
+/** Optional producer-declared {@link ContextForm}, mixed into the source shapes that carry one. */
+interface ContextFormed {
+  readonly form?: ContextForm
+}
+```
+
 ## Streaming
 
 Adapters emit a raw **chunk** protocol; the loop logs the chunks (replay fidelity) while feeding the same chunks through a `BlockAssembler` to rebuild blocks and messages. `StreamChunk` is a closed discriminated union over `type` — `block-start`, `text-delta`, `reasoning-delta`, `tool-call-delta`, `block-end`, `usage`, `finish`.