refactor(mcmc): resolve sampler_type once via resolve_sampler_spec

[MaartenMarsman]

What

config.sampler_type was string-compared at four sites: create_sampler (the factory), the learn_sd flag for the WarmupSchedule, and the has_nuts_diag/has_am_diag reservation flags. Adding a sampler meant editing each.

Adds resolve_sampler_spec(string) -> SamplerSpec{kind, learn_sd, nuts_diag, am_diag} as the single place that decodes the string. create_sampler now switches on the SamplerKind enum; run_mcmc_chain/run_mcmc_sampler read the spec fields.

Per the design-review critique, this is a free function — not a field on the thread-shared const SamplerConfig. bgmCompare, target_acceptance, and initial_step_size are untouched; create_sampler has no callers outside this translation unit, so the signature change is contained.

Why this is safe

Pure control-flow plumbing: same factory choice, same three flags, no RNG or math change.

Verified bitwise: adaptive-Metropolis GGM, NUTS GGM, and adaptive-Metropolis mixed-MRF raw draws are digest-identical pre/post. identical(main, branch) == TRUE. Harnesses: dev/bitwise_ggm_mh_primitive.R, dev/bitwise_ggm_nuts.R, dev/bitwise_mixed_logdet_ratio.R.

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.70%. Comparing base (e85bd6c) to head (3b37d9c).

Files with missing lines	Patch %	Lines
src/mcmc/execution/chain_runner.cpp	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
- Coverage   87.73%   87.70%   -0.03%     
==========================================
  Files          87       87              
  Lines       12853    12859       +6     
==========================================
+ Hits        11276    11278       +2     
- Misses       1577     1581       +4     

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