Add live scrub: keyboard and mouse seeking for pyflow play (TASK-048)

[AdamClemens]

Summary

Closes Stage 8's Completion Criterion 6 (see #83, #84, #85) -- the one
clause of the stage's own Goal ("scrubbed to any point") that shipped
without any mechanism at all.

• Left/Right step one frame; Home/End jump to the loaded window's own
  edges; a draggable scrub bar reaches any frame in between directly.
  Scoped to the window already materialized at launch.
• The task's biggest open risk resolved in favour of the primary
  approach, not the stated fallback: a scrub-bar drag must not also
  pan the camera, since RenderWindow.run() wires its own pan handlers
  on the same canvas. Read directly and confirmed live:
  rendercanvas's EventEmitter.emit dispatches handlers by order
  then registration order and stops once a handler sets
  event["stop_propagation"]. The scrub bar's own handlers register at
  order=-1, one level above the pan handlers' default order=0, so a
  drag on the bar suppresses panning for that gesture with no change
  to window.py needed at all.
• A new screen_to_world (rendering/window.py) -- the absolute
  screen-pixel-to-world mapping the bar needs for placing its thumb and
  hit-testing a drag, since _update_pan only ever tracked a delta --
  verified against a real rendered marker at a known world position
  before being trusted.
• Two real glfw-window integration tests, genuine injected keyboard and
  pointer events, prove both that seeking works and -- the literal
  claim this task exists to prove -- that the camera does not move at
  all during a scrub drag.

Stacked on #85 (feat/partial-overlap-cache-reuse, itself stacked
on #84 → #83). The diff will shrink as the earlier PRs merge.

Test plan

• Strict TDD throughout
• Empirical verification before relying on either finding (a live
  stop_propagation check, and a real-marker screen_to_world
  check) -- both recorded in docs/CHANGELOG-DESIGN.md
• make lint and every relevant make check-* target clean
• make test -- 1175 passed, 21 skipped, exit 0
• Verified by hand: a real offscreen render shows the scrub bar
  actually drawing (non-background pixels present); pyflow play --help unchanged (no new CLI flag, confirmed directly)

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