feat: per-preset tone curves via render/art-defaults payload (#32)

[tamirelazar]

Closes #32 (lever 5).

What

Establishes a per-preset render/art-defaults payload (RenderArtDefaults) and routes the existing intensity_mapping render parameter through it — without changing any output.

This is deliberately pure plumbing. Every preset's default tone curve stays at the current global log10; tasteful per-preset values are deferred to the showcase-presets issue (#36), which extends this same payload. The point of #32 is the clean render-layer seam that later render levers (#33 palette cycles, #6 gradient modes, #10 glyph selection) plug into without re-plumbing call sites.

Changes

• RenderArtDefaults { intensity_mapping } in src/render_art_defaults.rs, resolved per Preset via From<Preset> + Default (log10 for all presets).
• --intensity-mapping is now Option<String> (no clap default) so "absent → per-preset default" is distinguishable from "explicit override". Bare run still resolves to log10.
• Args::to_render_art_defaults() resolves the per-preset default, overriding only when the flag is explicitly set (mirrors the deposit-curve override pattern).
• The render call sites (runner.rs startup + four export/preview paths in app/mod.rs) read the resolved payload, preserving each site's existing error-handling semantics.
• intensity_mapping is a render parameter — it stays out of SimConfig.
• SavedConfig.intensity_mapping: Option<String> already round-tripped with serde back-compat; untouched.

Back-compat proof

• All 47 visual-regression goldens unchanged (no UPDATE_GOLDEN).
• Empirical same-seed capture (seed 42, 20 frames): default (Organic) and Vortex frames are byte-identical to origin/dev; --intensity-mapping linear differs from log (override works).

Test plan

• cargo fmt --check, cargo clippy --all-targets --all-features -- -D warnings — clean.
• cargo test (805 lib + 5 cli_integration + 47 visual_regression + 19 integration_app) — green.
• cargo test --all-features --lib (816) — green.