feat: combine the Julia and Mandelbrot screensavers into one paired view

[BernardJen]

Owner request: combine the Julia Set and Mandelbrot screensavers into one.

The idea came out of reviewing the (rejected) Julia rewrite on #122, which drew a
small Mandelbrot inset with a marker showing where the current c parameter sat.
That inset was the best thing in it: it made the boundary tour legible instead of
mysterious, because you could see why the Julia shape looked the way it did.

Combining the two turns that inset into the point of the saver rather than a corner
decoration — and the 5:1 wall is unusually well suited to showing two related images
side by side.

Why the two belong together

Every Julia set is defined by a single complex parameter c, and the Mandelbrot set
is precisely the map of which c values give a connected Julia set. They are two
views of the same object. Shown together, the relationship is self-evident:

• Move c along the Mandelbrot boundary and the Julia set morphs continuously.
• Sit c inside a bulb and the Julia set is connected; step outside and it shatters
  into dust. That transition is visible, and it is the single most interesting thing
  either saver can show.
• The current quality: Julia Set screensaver — tour the Mandelbrot boundary, kill the brightness pulse #122 asks for a "tour of the Mandelbrot boundary". This makes the tour
  observable rather than implied.

Composition options for 6000×1200

A — Split panes. Mandelbrot left, Julia right, with the c marker on the
Mandelbrot side. Simple, legible, and each pane is still 3000×1200, which is a
comfortable aspect. Recommended starting point.

B — Julia full-frame, Mandelbrot as a map inset. Closer to what the rejected
branch did. The Julia set gets the whole wall; the map sits in a corner as
orientation. Better if the Julia set is the subject and the map is a caption.

C — Continuous morph across the frame. c varies with horizontal position, so
the frame shows a family of Julia sets shading into one another. Visually striking
and unusual, but it stops being a tour and loses the map entirely.

A is recommended; B is a reasonable alternative. Pick one and record it here before
implementation.

Interaction with #121, which is in flight

This needs deciding before anyone writes code, because there is real tension:

#121's Mandelbrot rewrite has been reviewed and kept, and its whole premise is an
infinite dive by perturbation theory — a double-double reference orbit on the CPU
so the zoom can go past float precision. A dive is the opposite of a stable map you
can point at.

Two ways to reconcile them, and they are genuinely different products:

1. Dive with a following Julia. The Mandelbrot pane keeps diving, and c is
   taken from the dive centre, so the Julia pane morphs as the zoom descends. This
   preserves everything quality: Mandelbrot screensaver — make it an actual infinite dive (perturbation theory) #121 built and makes the pairing dynamic. More interesting,
   and more work.
2. Zoomed-out map, no dive. The Mandelbrot pane stays wide enough to be a usable
   map and c walks the boundary. Simpler, but it discards the perturbation work
   quality: Mandelbrot screensaver — make it an actual infinite dive (perturbation theory) #121 exists for — in which case quality: Mandelbrot screensaver — make it an actual infinite dive (perturbation theory) #121 should be closed as superseded rather than
   quietly abandoned.

Option 1 is recommended: it keeps the deep-dive work and produces a better saver
than either half alone.

What happens to the two existing savers

Combining reduces the rotation from 30 entries to 29. Decide explicitly:

• Replace both julia.js and mandelbrot.js with one combined module, or
• Keep Mandelbrot standalone (it is a strong saver in its own right, and quality: Mandelbrot screensaver — make it an actual infinite dive (perturbation theory) #121 is
  making it stronger) and replace only Julia with the combined view.

The second is probably better — a deep dive and a paired map are both worth showing,
and they are different enough not to feel like a repeat.

Acceptance criteria

• Composition (A/B/C) chosen and recorded here before implementation
• The quality: Mandelbrot screensaver — make it an actual infinite dive (perturbation theory) #121 interaction resolved and recorded: dive-with-following-Julia, or static map
• Decided and recorded whether Mandelbrot survives as a standalone entry
• The c marker is clearly visible and unmistakably tied to the Julia view
• Connected-to-dust transition visible during a normal ten-minute slot
• Both panes readable at 6000×1200; neither reduced to a thumbnail
• Readable at 12% washout — the rejected Julia branch was too dark, so this needs
  watching
• registry.js updated, and the entry count in any documentation updated with it
• Frame cost at 6000×1200 recorded — two fractal panes is roughly twice the
  fragment work of one
• Structure baseline added for the new entry, that saver's line only

[BernardJen]

Carrying one item across from #122, which is being closed in favour of this issue.

The brightness pulse is a defect, not a style preference, and it should not be lost. #122 recorded that the standalone Julia Set pulses in overall brightness as its parameter moves — the frame visibly brightens and dims rather than holding a stable exposure. On a videowall in a lit room that reads as the display malfunctioning, which is the same class of problem as #88's washout concerns.

It matters more in a paired view, not less: if the two panels pulse independently the eye reads them as two unrelated images rather than as one system showing a relationship. Whatever fixes exposure here should fix it for both halves together.

The rest of #122 — "tour the Mandelbrot boundary" — is subsumed by this issue's design, since a Julia parameter tracking a boundary point is exactly that, with the Mandelbrot shown alongside so the relationship is explicit instead of implied.