feat: add bounded file byte reads

docs/DEV_PLUGINS.md

@@ -85,12 +85,13 @@ Frontend bundles are mounted with a plugin-scoped API created by
 - `capabilities.list/get/has`
 - `commands.register/execute` for handlers declared in `contributes.commands`
 - `events.publish/subscribe` using the bundled frontend event bus
-- `files.list/metadata/readText/writeText/createFolder/move/trash/openExternal/showInFolder`
+- `files.list/metadata/readText/readBytes/writeText/createFolder/move/trash/openExternal/showInFolder`
   for canonical vault-relative slash paths guarded by `files.read`,
   `files.write`, `files.delete`, and `files.openExternal`. Backslashes,
   Windows absolute paths, UNC paths, traversal, `.verstak` variants, and
   symlink read/write/move/trash/external-open operations are rejected. Text
-  read/write is UTF-8 only and limited to 2 MB for reads.
+  read/write is UTF-8 only and limited to 2 MB for text reads; `readBytes`
+  returns base64 for regular files up to 8 MB.
 - `workbench.openResource/editResource` for routing vault resources to
   contributed `openProviders`. Plugins must declare `workbench.open`; this is a
   policy/contract check. Files and Notes plugins call this API and do not import

docs/PLUGIN_RUNTIME.md

@@ -437,6 +437,8 @@ contributions summary.
 - `files.list(relativeDir)` — list directory using a vault-relative path.
 - `files.metadata(relativePath)` — returns file/folder/symlink metadata.
 - `files.readText(relativePath)` — reads a UTF-8 regular file, with a size limit.
+- `files.readBytes(relativePath)` — reads a regular file up to 8 MB and returns
+  `{ relativePath, size, mimeHint, dataBase64 }` for bounded binary preview use.
 - `files.writeText(relativePath, content, options)` — atomically writes text via
   temp-file-and-rename. `options.createIfMissing` and `options.overwrite`
   control conflicts.
@@ -464,8 +466,9 @@ contributions summary.
 - `files.metadata` may report a final symlink as `type: "symlink"`, but
   `files.list` through a symlink directory and all read/write/move/trash
   operations through symlinks are forbidden in Milestone 6a.
-- Files API is text-only for read/write in Milestone 6a. `readText` is limited
-  to UTF-8 regular files up to 2 MB. Binary streaming is deferred.
+- Files API writes are text-only. `readText` is limited to UTF-8 regular files
+  up to 2 MB. `readBytes` is a bounded read-only byte contract up to 8 MB;
+  write streaming is deferred.
 - Live watcher refresh is active while Verstak is running and a vault is open.
   It performs an initial no-event snapshot, then publishes `file.changed` for
   external creates, updates, and deletes outside `.verstak/`. It does not keep a
@@ -540,6 +543,7 @@ bundled runtime. Это реальный runtime contract для cooperative bun
 | `api.files.list(relativeDir)` | ✅ Работает | Список vault-relative директории, `.verstak` скрыта |
 | `api.files.metadata(relativePath)` | ✅ Работает | Metadata для файла/папки/symlink без чтения содержимого |
 | `api.files.readText(relativePath)` | ✅ Работает | Читает UTF-8 regular file до 2 MB |
+| `api.files.readBytes(relativePath)` | ✅ Работает | Читает regular file до 8 MB как base64 payload |
 | `api.files.writeText(relativePath, content, options)` | ✅ Работает | Atomic text write с явным create/overwrite policy |
 | `api.files.createFolder(relativePath)` | ✅ Работает | Создаёт vault-relative folder |
 | `api.files.move(from, to, options)` | ✅ Работает | Move file/folder с conflict и path-policy checks |