Depth-resolved attenuation: 4 estimators, Li 2020 default

[FIrgolitsch]

Stacked PR 20/22 — review order: #115 → #97 → #98 → #99 → #100 → #101 → #108 → #106 → #107 → #87 → #116 → #110 → #111 → #40 → #112 → #113 → #117 → #118 → #120 → #121 → #122 → #123 → #124 → #125

Base: pr-s-metrics-stacking. Retargets to main as upstream PRs merge.

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Depth-resolved attenuation: 4 estimators, Li 2020 default

Stacked on pr-s-metrics-stacking. Adds Liu 2019 and Li 2020 depth-resolved attenuation estimators, makes Li the default, and exposes new CLI/Nextflow knobs.

Commits

• Add Liu 2019 and Li 2020 attenuation methods, rename Smith, fix Faber, document — introduces the family; linumpy/intensity/attenuation.py gets the four estimators sharing the Vermeer 2014 backbone.
• Refactor attenuation: shared helpers, optional GPU Vermeer core, shorter docstrings
• Document 4-method attenuation real-data sweep (Liu/Li ~3× flatter than Smith) — the empirical comparison that justifies the default change.
• Make Li 2020 the default attenuation method in pipeline and CLI — --method li is now the CLI default; compensate_attenuation_method = 'li' in nextflow.config.
• Doc update — minor follow-up to docs/ATTENUATION_METHODS.md.
• expose kernel/zshift/strength (squashed) — linum_compensate_attenuation.py gains --kernel, --zshift, --strength.

Full background, equations, and the real-data sweep are documented in docs/ATTENUATION_METHODS.md.