Dodge & burn as print exposure; density-driven chemical toning

[marcinz606]

Summary

Two physical-realism upgrades to the darkroom print model:

Dodge & burn in the print-exposure domain

• Masks no longer multiply the finished positive (2^strength, which clipped to flat black/white); they now offset per-pixel print exposure ahead of the H&D curve, in the same domain as WB filtration. Burns roll into paper black through the toe, dodges lift toward paper white through the shoulder — like holding back light under the enlarger.
• The separate local stage/shader is gone: the EV map (compute_local_ev_map) feeds the exposure pass on CPU and GPU, tiled export slices it per tile, and the exposure cache stage is keyed on (exposure, local).
• Dodge & Burn UI section moved to the Exposure tab; tutorial step moved and reworded.
• Fixed en route: exposure UBO binding size (240 → 256) for the grown uniform struct.

Density-driven selenium & sepia (B&W)

• Chemical toners act on the print's silver density (D = −log10(t)) instead of luma-masked RGB tints: selenium converts the densest silver first (Dmax boost — visibly deeper blacks — and cool eggplant shadows); sepia bleach-redevelops the thinnest silver first (warm highlights, shadows hold; partial strength = classic split-sepia). Paper white stays white.
• Runs on the linear buffer, no OETF bracket; CPU and WGSL share TONING_CONSTANTS. Conversion caps at all-silver-toned so the 0–2 sliders behave as a longer bath.
• Also gates GPU chemical toning to B&W, matching the CPU (it was silently active in colour modes).

Look change: existing edits using dodge/burn masks or selenium/sepia will render differently (noted in CHANGELOG under 0.34.0).

Test plan

• make all green (lint, ty, full pytest).
• GPU parity suite: 30/30 including dodge/burn through the new exposure path and new B&W chemical-toning parity cases (first B&W coverage in the suite).
• New unit tests: EV-map math (additivity, feather, geometry), burn/dodge roll-off vs clipping, toner density behaviour (densest/thinnest-first, paper-white identity, slider saturation).
• Verified live: drove the real app under Xvfb (.claude/skills/verify/SKILL.md has the recipe) — selenium deepens/cools shadows, sepia warms highlights only, −3 EV burn keeps tonal separation, Dodge & Burn section on the Exposure tab.