feat: add quantitative views, time controls, and finish dead wiring

[veillette]

The sim's physics and a11y were complete; what was missing was what a
learner can see and do with them.

Quantitative views

• IntensityProfileNode: a bamboo trace of intensity across a cut through
  the detector, on Michelson and both Mach-Zehnder ports. Turns the
  photograph into a measurement — the modulation depth is the visibility,
  the ripple count is the fringe count. With both ports plus the dashed
  total it draws I_A + I_B = const, which doc/model.md calls the point of
  that screen and which two percentages could only imply.
• CoherenceEnvelopeNode: visibility against path difference on Michelson,
  with a marker at the current OPD. Makes doc/model.md §2 visible — flat
  for a laser, a needle for white light, and the sodium doublet's nulls
  and revivals. The span comes from the source's own feature scale, so
  metres and micrometres are both legible.
• Both draw through the existing physics: a new pure intensityProfile()
  beside intensityAt(), and spectrumVisibility() unchanged.

Time controls

• TimeModel (previously unused) is composed into the two models that
  evolve on their own, each with stepOnce() for single-frame advance.
  Photon accumulation can now be paused and stepped a few photons at a
  time; the Fabry-Pérot sweep can be stopped on a transmission peak. Its
  scan checkbox is replaced by the clock rather than joined to it.

Finished features that were built but never reached the view

• showOpticalPath preference: plumbed from main.ts to the table nodes,
  labelling each element with what it adds to the path difference. The
  Michelson's contributions are doubled and the Mach-Zehnder's are not,
  which makes the factor of two concrete.
• Mach-Zehnder vertical mirror tilt, which had a model Property and an
  a11y string but no control; and its visibility readout.
• Path difference in wavelengths, using the unused units.waves pattern.
• Home-screen icons, which were three blank rectangles.

Also

• New charts carry accessibleParagraph descriptions derived from the same
  samples they draw; all strings added in en/es/fr.
• Fixed a leak the memory-leak suite caught: a chart must dispose the
  formatter Properties behind its description, not just its Multilink.
• CSP: allow the two inline onclick handlers scenery writes on disabled
  controls, scoped to script-src-attr by hash. Without this the step
  button logs a violation on every enable/disable and fails the fuzz run.

138 tests (was 124), including that the two ports' profiles sum to a
constant and that the sodium null falls inside the mirror's travel.

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✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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