docs: finish the whole-file-diff sweep across comments and RFCs

Codex's stale-prose pass found seven more spots still describing the
result diff as ALWAYS an applied contextual hunk, or a create/binary
overwrite as rendering "only the call-time card": the DiffCallView JSDoc
and the acp bridge diff-arm comment, the FsWriteOutcome.before and
readTextForDiff JSDoc, and three RFC lines. All now say: the result diff
is the applied change — a contextual hunk when there is a before-image,
else a whole-file diff (create / undiffable binary) — and a successful
mutation always returns the result diff so the model-facing text can't
clobber it. Regenerate the cordis catalog (source line shift).

packages/fs/fs-local/src/fsio.ts

@@ -386,8 +386,9 @@ export async function readForEdit(
  * Best-effort read of a file's current text for a before/after diff basis, used
  * by an overwrite. Returns the LF-normalized decoded content, or `null` when the
  * file is binary or not valid UTF-8 — a write must succeed regardless of the
- * prior bytes, so an undiffable prior file simply yields no contextual diff
- * (the caller treats `null` the same as an absent file: call-time card only).
+ * prior bytes, so an undiffable prior file simply yields no contextual-hunk basis
+ * (the caller treats `null` the same as an absent file: the result renders a
+ * whole-file diff rather than an applied hunk).
  */
 export async function readTextForDiff(absolutePath: string, signal?: AbortSignal): Promise<string | null> {
   const buffer = await readFileAbortable(absolutePath, 'read', signal)

packages/fs/fs/src/types.ts

@@ -103,8 +103,10 @@ export interface FsWriteOutcome {
   version: FsVersion
   /**
    * The file's content BEFORE the write, or `null` when the file did not exist
-   * (a create). LF-normalized storage text (the diff basis), never a diff — a
-   * consumer computes the result-time contextual diff from `before`/`after`.
+   * (a create) or was undiffable (binary/non-UTF-8). LF-normalized storage text
+   * (the diff basis), never a diff — a consumer computes the result-time
+   * contextual diff from `before`/`after` when `before` is present, else falls
+   * back to a whole-file diff.
    */
   before: string | null
   /** The file's content AFTER the write, LF-normalized to share `before`'s diff basis. */