mitosis: fix vtime drift for borrowed and cross-cell-pinned tasks

[likewhatevs]

Problem

scx_mitosis charges each task's dsq_vtime on every stop, but only
advanced the owning cell's vtime_now for in-cell, un-borrowed runs.
Runtime a task spent borrowing an idle CPU from another cell, or
pinned to a CPU outside its home cell, was charged to dsq_vtime
but hidden from every cell's vtime_now — the basis the next
mitosis_enqueue compares against and the value mitosis_dispatch
orders by.

So dsq_vtime drifts away from every basis it is ever checked against.
For a SCHED_IDLE task (weight 1 → charged at 100×) the gap grows fast
and surfaces two ways:

• "vtime too far ahead" error: dsq_vtime exceeds the basis by more
  than 8192 * slice_ns (~164s at the 20ms default), so mitosis_enqueue
  calls scx_bpf_error and the scheduler exits.
• Runnable-task stall: before the gap gets that large, the drifted
  task already loses every vtime-ordered dispatch and the watchdog fires
  (SCX_EXIT_ERROR_STALL).

Fix

Both behaviors are gated behind --vtime-borrow-fixes (default off, so
this is a no-op until the flag is set).

• Borrowed runtime: mitosis_stopping advances the task's
  charge-cell vtime_now regardless of borrow / CPU retag, so a
  borrowing task keeps its home cell tracking the runtime it spent on loan.
• Cross-cell pin: a task pinned to a CPU outside its home cell is
  charged to the cell of the CPU it actually runs on — vtime_charge_cell
  is taken from the run CPU in mitosis_select_cpu and mitosis_enqueue,
  not from the home cell. That cell and that CPU's vtime_now then track
  it, so its dsq_vtime stays in the domain the foreign cell's tasks —
  and mitosis_dispatch — compare against, and it stays dispatchable.

Commits

1. add ktstr reproducers for vtime drift errors and stalls — four
   KVM-VM reproducers (error + stall, each via borrowing and via
   cross-cell pin), the --vtime-borrow-fixes flag (declared as a no-op
   here), and the ktstr dev-dependency behind the ktstr-tests feature.
   With the flag a no-op, all four reproduce the bugs (fail) at this commit.
2. fix vtime drift for borrowed and cross-cell-pinned tasks — adds
   the vtime_borrow_fixes behavior; with the flag set, the four
   reproducers pass.

Test plan

Each reproducer was run against both commits (--kernel ../linux,
-E 'test(mitosis_vtime)'):

• commit 1 (flag a no-op): all four reproduce the bug — 2/2 runs
• commit 2 (fix): all four pass — 2/2 runs

note: stall_via_borrowing is flaky, 1/4 runs errantly passes without fix. this is a hard bug to reliably reproduce.