world clock C5: SimThread, NativeSimThread and the demo pacer follow a WorldClock

[transfix]

The Python side of "every simulation on one world clock"
(libcvc docs/roadmap/WORLD-CLOCK-SYNC-ROADMAP.md, step C5). It builds on the
clock parity work in #116.

clock.WorldClock:

• a Condition notified on every tick change;
• wait_for_tick_change(tick, timeout), so a follower thread sleeps until the
  driver moves the clock instead of polling (as cvc::world_clock does).

sim_thread.SimThread:

• SimThread(world, clock=clock) FOLLOWS a shared clock: one step every
  world.dt / fixed_dt quanta, run as the driver advances it, as
  cvc::nav::sim_thread does. Pause skips this sim's due steps; the clock's
  own pause pauses everything.
• SimThread(world, hz=60) keeps its old behaviour on a private clock (quantum
  world.dt, speed hz * dt) that the thread drives. Pause, SetRate and Nudge
  act on that clock, so world time and the steps run always agree.
• wait_caught_up(timeout), so a capture shows exactly each frame's steps.

nav_native.NativeSimThread(world, clock=app.world_clock()) is the C++ worker
following a shared pycvc clock, plus wait_caught_up. It needs the pycvc
functions from transfix/libcvc#630; HAS_SIM_THREAD_CLOCK gates it.

demos._common.SimPacer keeps its API but now drives a WorldClock at the
suite quantum (0.01 s), with a ClockFollower for the navigator. It was a
private accumulator; its .clock is exposed for a host. A stall is capped at
8 navigator steps, and the clock drops and counts the rest.

Tests, run against a local libcvc build with #630:

• test_clock: the tick wait wakes on advance, times out without one, and
  counts a rewind as a change;
• test_swarm: SimThread on a shared lockstep clock runs tick // k steps on
  every frame, and none while paused;
• test_sim_thread: NativeSimThread on app.world_clock(), the same check on the
  C++ worker;
• test_sim_pacer: frame-rate independence, speed and the stall drop still
  hold.

All 19 tests in the clock, sim_thread and SimThread selection pass, and so do
test_sim_pacer and test_clock (17). ruff 0.16 and black 26.5.1 are clean.