feat: add a Length Contraction screen (ladder and barn)

[veillette]

Length contraction was the one standard topic the sim deliberately left out,
and the repo's own docs flagged a ladder-and-barn treatment as the natural
next screen. This adds it, third of five, after the Spacetime Diagram screen
that introduces the simultaneity it is resolved by.

A ladder of proper length L0 flies at beta through a barn whose two doors are
wired to one switch. The two slams are events at ( -B/2, 0 ) and ( +B/2, 0 )
in the barn frame: spacelike separated for every non-zero barn, so no frame's
answer to "which shut first?" is the wrong one. The barn frame sees the ladder
contracted to L0/gamma and briefly wholly inside; the ladder frame sees the
barn contracted to B/gamma, the exit door shutting and reopening while the
nose is still short of it, and the entrance door shutting behind the tail
gammabetaB later. No door ever touches the ladder in either account.

The screen's central control is the frame selector. Flipping it rearranges the
stage completely while changing exactly one thing on the spacetime diagram
below - the tilt of the simultaneity slice the measurement is taken along -
because the diagram is always drawn in barn-frame coordinates and the two
slam markers do not move. Each object's ends sweep out a world sheet, so
"does it fit?" becomes visibly a question about which slice of the overlap
you take.

Also here:

• both frames' clocks are zeroed on one shared event (the centres crossing),
  which is what makes a single scene-time number meaningful across the toggle;
• no scrubber - at high beta the window is set by the slams rather than the
  fly-past, so a fixed-range slider would have had a few percent of useful
  travel. Two "go to slam" buttons replace it, and teach better: in the barn
  frame they land on the same instant;
• createPushButton takes an optional enabledProperty, and controlHelpers gains
  createRadioButtonGroup, so the new controls are the shared ones;
• 22 new tests, including each frame's fitting verdict re-derived by sweeping
  the snapshots the view actually draws, and the check that no door shuts
  through the ladder;
• en/es/fr strings, a11y summary, keyboard help, and screen icon;
• model.md, implementation-notes.md, CLAUDE.md and README updated.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01D861mRUkK82cR8wuSLwtij

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