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.

docs/cordis-catalog/services.md

@@ -481,7 +481,7 @@ Abstract directory-picking service. Subclass, implement `capability()`, and load
 abstract capability(): DirectoryPickerCapability
 ```
 
-Source: [`packages/host/directory-picker/src/index.ts:120`](../../packages/host/directory-picker/src/index.ts)
+Source: [`packages/host/directory-picker/src/index.ts:121`](../../packages/host/directory-picker/src/index.ts)
 
 ## `ctx.fs` — `FileSystem` (abstract seam)