fix(host,client): require fully qualified browse paths; clear the picker kind on close

ds-review-bot round 4. On Windows, isAbsolute admits rooted drive-less
forms (\foo, /foo) that resolve() then rebases onto the process's current
drive; both browse primitives now gate on a fullyQualified check (drive
letter or UNC on win32, POSIX-absolute elsewhere) with a platform test
seam, per-platform unit cases, and the contract wording updated on the
seam, the backend README pair, and the error messages.

The picker-kind effect also kept a resolved 'dialog' across close, so a
backend swapped while the menu was closed could paint the stale entry for
one frame on reopen; the close arm now clears the state, pinned by a
reopen-under-pending-read race test.

packages/host/directory-picker/src/index.ts

@@ -60,8 +60,9 @@ export interface DirectoryPickerBrowseCapability {
    * List one directory level.
    * @param path - absolute directory to list; absent lists the home directory.
    * @returns the level's listing with ancestry.
-   * @throws {DirectoryPickerError} `directory-unreadable` when the target is not absolute
-   * (a wire value must never rebase under the host cwd) or cannot be listed.
+   * @throws {DirectoryPickerError} `directory-unreadable` when the target is not fully
+   * qualified (a wire value must never resolve against the host cwd or, on
+   * Windows, its current drive) or cannot be listed.
    */
   list(path?: string): Promise<DirectoryListing>
   /**
@@ -70,7 +71,7 @@ export interface DirectoryPickerBrowseCapability {
    * @param name - single non-blank path segment (no separators, not `.`/`..`).
    * @returns the created directory's absolute path.
    * @throws {DirectoryPickerError} `directory-exists` for an existing child,
-   * `directory-create-failed` for a non-absolute parent or any other failure.
+   * `directory-create-failed` for a parent that is not fully qualified or any other failure.
    */
   createDirectory(path: string, name: string): Promise<string>
 }