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- The superseded-in-part bullet states the affected clauses as one rule rather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.

scripts/type-equiv.manifest.json

@@ -31,6 +31,11 @@
       "symbol": "ContextForm",
       "source": "packages/llm/llm/src/message.ts"
     },
+    {
+      "doc": "docs/core-data-structures/core.md",
+      "symbol": "ContextFormed",
+      "source": "packages/llm/llm/src/message.ts"
+    },
     {
       "doc": "docs/core-data-structures/core.md",
       "symbol": "FinishReasonMap",