perf(shader): skip main bulbs, detect periodic orbits, and halve orbit loads

[rowan-m]

Summary

• Analytic main-cardioid & period-2 bulb check (in_main_bulbs): At shallow zooms (zoom > 1e-3, where f32 resolves $c = X_1 + \Delta c$ well below a pixel), skip pixels inside the main cardioid and period-2 bulb before entering the iteration loop.
• Brent periodicity detection in fs_main_f32: At shallow zooms (zoom > 1e-3), detect periodic interior orbits (such as period-3/4 bulbs and shallow minibrots) when $Z_n$ returns within zoom * 1e-5 of a saved checkpoint.
• Single reference_orbit load per iteration: Carry $X_{m+1}$ forward as the next iteration's $X_m$ (resetting to $X_0 = 0$ on rebase) across all three shader tiers (fs_main_f32, fs_main_ds, fs_main_qs), halving storage-buffer reads per step.

Pixel 11 Benchmark (1078×2008 canvas, fractious:full-res)

View	main	This PR	Speedup
Seahorse Valley + main bulb (x=-0.768..., y=0.212..., z=0.936, r=2.7)	10,945 ms (653 slices)	401 ms (11 slices)	~27×
Default view (x=-1.7, y=0, z=-0.301, iter=7500)	~10,000 ms	268 ms (14 slices)	~37×
Whole Mandelbrot set (x=-0.5, y=0, z=-0.301, iter=7500)	6,766 ms	189 ms (10 slices)	~36×

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