fix(fs-local): bound overwrite contextual diff bases

Rebuild of the fs-overwrite-diff-bound branch on current master. Adds the
diffBasisMaxBytes Config field (10 MiB default, capped by runtime
allocation/decode limits), gates both overwrite sides, and reads the prior
basis from the bounded opened descriptor in cancellation-aware chunks; any
post-stat size change returns a null basis. Also pins the one-extra-byte
growth probe with a regression and drops the now-covered v8 ignore.

docs/core-data-structures/filesystem.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/core-data-structures/filesystem.md
-filesystem.md: 110c1fd428b15c5094f9dcc94050cad61c324373
-filesystem.zh.md: 010ec22a5d4a29555425c3deede08b317cba7004
+filesystem.md: 4862a25e3b8922a8709b4b025acd5436a2228082
+filesystem.zh.md: 2b5c7abec1555cc2cbb909eaae60d9022ae297dd

docs/core-data-structures/filesystem.md

@@ -134,10 +134,11 @@ interface FsWriteOutcome {
   version: FsVersion
   /**
    * The file's content BEFORE the write, or `null` when the file did not exist
-   * (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.
+   * (a create) or the backend declined a contextual basis (for example, a
+   * binary/non-UTF-8 prior file or either overwrite side reaching its exclusive limit).
+   * 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. */

docs/core-data-structures/filesystem.zh.md

@@ -134,10 +134,11 @@ interface FsWriteOutcome {
   version: FsVersion
   /**
    * The file's content BEFORE the write, or `null` when the file did not exist
-   * (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.
+   * (a create) or the backend declined a contextual basis (for example, a
+   * binary/non-UTF-8 prior file or either overwrite side reaching its exclusive limit).
+   * 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. */