docs(core): document ContextFormed as the discriminated union it became

The rebase carried the simple-interface block from the base branch, while
this branch turned ContextFormed into a per-form discriminated union;
ContextSnapshotSection gets its own block so the page stays self-contained.

docs/core-data-structures/core.md

@@ -202,12 +202,43 @@ type ContextForm =
 ```
 
 ```ts type-equiv
-/** Optional producer-declared {@link ContextForm}, mixed into the source shapes that carry one. */
-interface ContextFormed {
-  readonly form?: ContextForm
+/** One named contribution to a `snapshot`-form context, in assembly order. */
+interface ContextSnapshotSection {
+  /** The contributing subsystem's name. */
+  readonly name: string
+  /** That contribution's model-facing text, exactly as assembled. */
+  readonly text: string
 }
 ```
 
+```ts type-equiv
+/**
+ * Producer-declared {@link ContextForm} and the fields that form requires,
+ * mixed into the source shapes that carry one.
+ *
+ * Discriminated by `form` so a producer cannot declare a shape without the
+ * facts that shape is presented from: a `notice` must record its one-line
+ * account, a `snapshot` its sections. Omitting `form` stays valid — an
+ * undeclared context is the documented default.
+ */
+type ContextFormed =
+  | { readonly form?: never }
+  | { readonly form: 'instructions' }
+  | { readonly form: 'catalog' }
+  | {
+    readonly form: 'snapshot'
+    /** The named contributions this snapshot assembled, in order. */
+    readonly sections: readonly ContextSnapshotSection[]
+  }
+  | {
+    readonly form: 'notice'
+    /** One-line account of what happened, shown without expanding the row. */
+    readonly summary: string
+  }
+  | { readonly form: 'relay' }
+  | { readonly form: 'recall' }
+```
+
 ## 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`.