fix(terminal): end the page-key debt lifetime once travel repays it

pageKeysForGesture kept `prepaid` set after the travel had fully repaid a
pre-paid page, so the one-second debt lifetime still applied: on a 38-row
pane, 19 lines of -1 at 32 ms left { pending: 0, prepaid: true } and a -1
300 ms later sent nothing. `prepaid` now clears as soon as the pending
travel reaches or crosses zero, so the leftover is plain travel and the
150 ms gap applies again. Touch contacts are unaffected (never split by
time). The mixed parity step's reversal grows from 20 to 25 rows because
the -5 rows left after repayment are now plain travel and cancel against it.

test/terminal-scroll-routing.test.ts

@@ -557,6 +557,56 @@ describe('page-key gestures across slow touch drags, slow wheel ticks and jitter
     expect(p.keys()).toBe(UP + DOWN);
   });
 
+  it('ends the one-second debt lifetime once the debt is repaid, exactly or overpaid', () => {
+    // 38 rows: 19 a page. The opening -1 pre-pays and owes 18; 18 more lines at
+    // 32 ms repay it EXACTLY. Nothing is owed any more, so the 150 ms gap applies
+    // again and a -1 300 ms later is a new gesture that pages at once.
+    const exact = pane();
+    for (let i = 0; i < 19; i++) {
+      exact.app._maybePageCliTranscript({ shiftKey: false }, -1);
+      exact.advance(32);
+    }
+    expect(exact.keys()).toBe(UP);
+    expect(exact.app._pageKeyGesture).toMatchObject({ pending: 0, prepaid: false });
+    exact.advance(300 - 32);
+    exact.app._maybePageCliTranscript({ shiftKey: false }, -1);
+    expect(exact.keys()).toBe(UP + UP);
+
+    // Overpaid: 19 lines after the opening one leave 1 row of plain travel.
+    const over = pane();
+    for (let i = 0; i < 20; i++) {
+      over.app._maybePageCliTranscript({ shiftKey: false }, -1);
+      over.advance(32);
+    }
+    expect(over.keys()).toBe(UP);
+    expect(over.app._pageKeyGesture).toMatchObject({ pending: -1, prepaid: false });
+    over.advance(300 - 32);
+    over.app._maybePageCliTranscript({ shiftKey: false }, -1);
+    expect(over.keys()).toBe(UP + UP);
+
+    // One line short of repaid: still owed, so 300 ms later is the same gesture.
+    const owed = pane();
+    for (let i = 0; i < 18; i++) {
+      owed.app._maybePageCliTranscript({ shiftKey: false }, -1);
+      owed.advance(32);
+    }
+    expect(owed.app._pageKeyGesture).toMatchObject({ pending: 1, prepaid: true });
+    owed.advance(300 - 32);
+    owed.app._maybePageCliTranscript({ shiftKey: false }, -1);
+    expect(owed.keys()).toBe(UP);
+  });
+
+  it('keeps a touch contact one gesture after its debt is repaid', () => {
+    const p = pane();
+    for (let i = 0; i < 19; i++) {
+      p.app._maybePageCliTranscript({ shiftKey: false, touchId: 5 }, -1);
+      p.advance(32);
+    }
+    p.advance(2_000); // the finger rests, still down
+    p.app._maybePageCliTranscript({ shiftKey: false, touchId: 5 }, -1);
+    expect(p.keys()).toBe(UP); // accumulates; no fresh pre-pay inside the contact
+  });
+
   it('logs a wheel the dialog check consumed', () => {
     const p = pane();
     p.app.tabAlerts = new Map([['sess-1', 'action']]);
@@ -681,7 +731,7 @@ describe('the paging gates asked for another pane (a TerminalTile)', () => {
       [-1, {}],
       [-10, {}],
       [-30, {}],
-      [20, {}], // a reversal inside the gap pages by travel, not at once
+      [25, {}], // a reversal inside the gap pages by travel (net of the -5 plain travel left), not at once
       [-5, { ctrlKey: true }],
       [-5, { deltaX: 400, deltaY: -125 }],
       [-1000, {}],