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/use-copy-feedback.ts

@@ -0,0 +1,37 @@
+// The copy-to-clipboard-with-feedback hook shared by the block primitives
+// (TerminalBlock, SearchBlock): write the given text, and on success flip a
+// transient `copied` flag that the caller renders as a "复制成功" label for one
+// second. A refused write leaves the flag untouched, so the control never claims
+// a copy the host declined.
+
+import { useCallback, useState } from 'react'
+import { writeClipboard } from './clipboard.ts'
+
+/** How long the `copied` flag stays true after a successful write, in ms. */
+const COPIED_FEEDBACK_MS = 1000
+
+/** The copy-feedback hook's return: the transient flag and the copy handler. */
+export interface CopyFeedback {
+  /** True for {@link COPIED_FEEDBACK_MS} after a successful write; render the success label off it. */
+  copied: boolean
+  /** Copy the hook's text; no-op while `copied` is still true, silent on a refused write. */
+  onCopy: () => void
+}
+
+/**
+ * Copy `text` to the clipboard with one-second success feedback.
+ * @param text - the text to write on copy.
+ * @returns the `copied` flag and the `onCopy` handler.
+ */
+export function useCopyFeedback(text: string): CopyFeedback {
+  const [copied, setCopied] = useState(false)
+  const onCopy = useCallback(() => {
+    if (copied) return
+    void writeClipboard(text).then((ok) => {
+      if (!ok) return
+      setCopied(true)
+      window.setTimeout(() => { setCopied(false) }, COPIED_FEEDBACK_MS)
+    })
+  }, [copied, text])
+  return { copied, onCopy }
+}