refactor(ui-primitives): extract shared head/tail cap and copy-feedback helpers

The tail-header cap fix pushed SearchBlock's head/tail slicing arithmetic and
its copy-feedback hook over the duplication gate's threshold against the
byte-identical logic in TerminalBlock. Extract both into head-tail-cap.ts
(headTailCap) and use-copy-feedback.ts (useCopyFeedback) and consume them from
both blocks, deleting the clone rather than nudging it under the limit.

packages/client/ui-primitives/src/head-tail-cap.ts

@@ -0,0 +1,33 @@
+// Head/tail height-cap arithmetic shared by the block primitives (TerminalBlock,
+// SearchBlock) and matching the TUI transcript's collapsed tool card, so a long
+// result's head and tail slices agree across every surface. The split is
+// `ceil(maxLines / 2)` head rows and the remainder as tail rows; a result within
+// the cap shows every row and hides none.
+
+/** The head/tail split metrics for a capped list. */
+export interface HeadTailCap {
+  /** Rows beyond the cap (list length − maxLines); ≤ 0 means nothing is hidden. */
+  hidden: number
+  /** Whether the list is over the cap and not expanded, so it shows a head/tail slice. */
+  capped: boolean
+  /** Head-slice row count: `ceil(maxLines / 2)`. */
+  headLines: number
+  /** Tail-slice row count: the remainder after the head. */
+  tailLines: number
+}
+
+/**
+ * Compute the head/tail cap metrics for a list of `total` rows against `maxLines`,
+ * given whether the surface is expanded. Pure arithmetic; the caller slices its
+ * own rows with `headLines`/`tailLines` so a block can layer its own concerns
+ * (SearchBlock restores a tail file header) on top.
+ * @param total - the list's row count.
+ * @param maxLines - the collapsed-height cap in rows.
+ * @param expanded - whether the surface is expanded (uncaps the list).
+ * @returns the split metrics.
+ */
+export function headTailCap(total: number, maxLines: number, expanded: boolean): HeadTailCap {
+  const hidden = total - maxLines
+  const headLines = Math.ceil(maxLines / 2)
+  return { hidden, capped: hidden > 0 && !expanded, headLines, tailLines: maxLines - headLines }
+}