Add ld_loader() for on-demand block-wise LD retrieval

[gaow]

Summary

• Add ld_loader() function factory for lazy, on-demand LD block loading
• Moved from susieAnn/R/ld_loader.R to share across packages (susieAnn, OTTERS pipeline, etc.)
• Three modes: pre-loaded R matrices, pre-loaded genotype X matrices, or on-the-fly loading via load_LD_matrix() from metadata TSV
• Optional max_variants subsampling for memory control
• susieAnn updated to re-export from pecotmr (separate commit in susieAnn repo)

Usage

# Pre-loaded LD
loader <- ld_loader(R_list = list(R1, R2, R3))
loader(1)  # returns R1

# On-the-fly from pecotmr metadata
loader <- ld_loader(ld_meta_path = "ld_meta.tsv",
                    regions = c("chr1:1-1000000", "chr1:1000001-2000000"))
loader(1)  # loads LD for first region

Test plan

• R CMD check passes
• susieAnn vignettes still work with re-exported ld_loader()

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