Part 3: add expanded nonparametric algorithms

[Siel]

This is part 3 of the split of #260 and is stacked on #264.

Scope:

• add the newer nonparametric algorithm families on top of the unified-platform base
• add the associated benchmark and comparison examples
• keep this review focused on the nonparametric algorithm expansion only

Why:

• separate the expanded nonparametric work from both the platform rewrite and the parametric stack
• keep the review surface aligned with the algorithm family being introduced

Validation:

• cargo check --tests --examples

[github-actions]

Bencher Report

Branch	split/unified-platform-nonparametric-pr
Testbed	mhovd-pgx

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Benchmark	Latency	milliseconds (ms)
bimodal_ke_npag	📈 view plot ⚠️ NO THRESHOLD	4,850.40 ms
bimodal_ke_npod	📈 view plot ⚠️ NO THRESHOLD	1,328.00 ms
bimodal_ke_postprob	📈 view plot ⚠️ NO THRESHOLD	459.71 ms

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