Add color engine, Mandelbulb 3D explorer, render resolution, and cursor-lock zoom

[Jasonilization]

Summary

• Render Resolution picker (0.5x-2x internal scale) for the live canvas, plus a selectable Save Image export resolution.
• Lock Cursor to Detail toggle: manual scroll/pinch zoom snaps its pivot onto nearby detailed structure instead of requiring pixel-perfect aim.
• Color engine: escape-time (smooth/banded)/distance-estimation/orbit-trap (circle/line/cross/custom) color modes, a relief-shading pass, and a full Palette Editor (add/remove/reposition stops, HSB + color picker, save custom palettes) -- all implemented consistently across the float32, double-double, and CPU perturbation tiers, plus new Classic Rainbow/Ice/Neon/Monochrome/Scientific presets.
• Mandelbulb Explorer: a new tab alongside the 2D explorer with a real-time ray-marched 3D Mandelbulb -- orbit camera, quality tiers, lighting/AO/soft shadows, power/variant presets, and screenshot export.
• GitHub profile link + Help sheet (already present from a prior commit) verified intact through the new tab-bar restructuring.

Test plan

• swift build clean, no errors
• Full existing preset + zoom-depth-ladder regression (PRESET_PREVIEW_DIR) -- all 24 renders across all 3 precision tiers succeed, no regressions
• New COLOR_MODE_TEST_DIR headless diagnostic -- every color mode/orbit trap shape/shading combination verified visually correct on both GPU and perturbation tiers (caught and fixed 4 real bugs along the way: gradient-cycle-speed regression, distance-estimation's absolute-epsilon clamp breaking at deep zoom, and orbit trap's series-approximation-skip derivative/baseline seeding)
• New MANDELBULB_TEST_DIR headless diagnostic -- power/variant/quality combinations render correctly
• Verified end-to-end in the actual windowed app: launched the built binary, used the accessibility API to switch between the Mandelbrot and Mandelbulb tabs and back, confirmed both render correctly and state is preserved
• Built and launched the packaged, ad-hoc-signed .app release bundle successfully

🤖 Generated with Claude Code

[Jasonilization]

Added in this update:

• Julia Set Explorer — a new tab sharing the exact same rendering engine as the Mandelbrot tab (GPU tiers, CPU perturbation with series approximation, progressive rendering, color pipeline, export, recording). Double-click any point in the Mandelbrot tab (or use the sidebar button) to open its Julia set. Julia sidebar adds live-updating c controls, Random Julia, presets, and persisted favorites.
• Progressive rendering fix — the CPU perturbation tier used to briefly flash a flat interior-colored frame whenever its render resolution changed (start/end of every interaction). It now seeds the resized texture with a bilinear resample of the previous frame instead, so movement always shows a continuous (if briefly blurry) image rather than a black flash.

Verified via headless diagnostics (full Mandelbrot preset/zoom-depth ladder to 1e80 across all three tiers, new Julia-mode GPU/perturbation tests, and a direct arbitrary-precision cross-check of the Julia perturbation math) plus manual GUI testing of tab switching, the Julia sidebar controls, and the Mandelbrot→Julia double-click handoff.

[Jasonilization]

Follow-up fix on top of the previous update: found and fixed a real correctness bug in the interactive-zoom quality reduction.

The bug: while panning/zooming, maxIterations is temporarily cut to keep frame rate smooth. A point that hasn't escaped by that cut budget might be genuinely interior, or might just need more iterations than the current frame allows — those two cases were indistinguishable, and the renderer guessed "interior," painting the flat interior color. At real zoom depth this reads as incorrect black holes punched into boundary detail during any interaction. Verified numerically: cutting the budget from 2000 to 250 iterations at a typical deep-zoom boundary view wrongly classified 100% of the genuinely-exterior pixels needing more than 250 iterations as solid interior.

The fix: a previewMode flag (threaded through both GPU kernels and the CPU perturbation path) that colors an unresolved point the same as an escaping point at the iteration cap instead of guessing "interior" — it blends into the surrounding gradient and resolves correctly once a full-iteration pass completes. Also changed the animated iteration cut from a fixed number (220/300) to one proportional to the view's actual maxIterations with a floor, so deep zooms keep meaningfully more live detail during interaction instead of collapsing to a near-fixed floor regardless of depth.

Added a PREVIEW_MODE_VERIFY headless regression check that renders the same view at full/cut-without-fix/cut-with-fix and reports the misclassification count — confirms 0 wrongly-classified pixels with the fix vs. 100% without. Also reran the full existing regression suite (preset/zoom-ladder to 1e80, color-mode tests) with no changes to full-quality output.