review: address ds-review-bot findings on composer geometry

- Rewrite the 08-04 gutter-reservation note in place: the reservation is Chat's alone and the overlay branch points to the seat-width compensation note; the Testing section now describes the uncompensated control.
- Cross-reference the earlier rejection of the seat inset in the 08-12 note and record the engine-resolved-vs-fixed-8px drift as accepted residual cost instead of asserting Firefox resolves 8px.
- Pair --dsh-scrollbar-width with the mirrored ::-webkit-scrollbar rule and the compensation consumer in the scrollbar-styles gate; document the variable in the ui-theme README.
- Rename the e2e control to compareTabsWithoutCompensation and correct the vacuity-guard comment; rewrap the WorkspaceBrowser doc comment.

packages/client/ui-workspace/src/client/WorkspaceBrowser.tsx

@@ -4,10 +4,10 @@
  * workspace), search, the grouped tree or flat list, and the workspace
  * dialogs. Wide state renders the full browser; rail state renders the two
  * region icons (search / add workspace) as 36px controls on the shell's shared
- * rail entry path, each requesting expansion through the owner share. Adding is the header
- * button's one action, so it raises the directory flow with no menu in
- * between; the flow and its error dialog live in WorkspacePicker (same
- * package — direct composition, no slot between them).
+ * rail entry path, each requesting expansion through the owner share. Adding
+ * is the header button's one action, so it raises the directory flow with no
+ * menu in between; the flow and its error dialog live in WorkspacePicker
+ * (same package — direct composition, no slot between them).
  */
 import { useEffect, useMemo, useRef, useState } from 'react'
 import clsx from 'clsx'