Add dig_itemsets() with dedicated R/C++ mining path and docs/tests wiring

[Copilot]

This PR adds first-class itemset mining alongside existing association-rule mining by introducing dig_itemsets() and routing it through a dedicated C++ storage/dispatch path. It mirrors the dig_associations() digging flow where appropriate, with itemset-specific API shape and outputs.

• R API: new dig_itemsets() surface

  • Added R/dig_itemsets.R with itemset-oriented arguments (items instead of antecedent; no consequent, min_coverage, min_confidence, contingency_table, threads).
  • Reuses .dig() pipeline with call_function = "dig_itemsets" and itemset semantics (focus = c(), filter_empty_foci = FALSE).
  • Returns nugget flavour itemsets with call metadata aligned to existing digger APIs.

• Dispatcher integration (.dig + Rcpp entrypoints)

  • Extended .dig() dispatch in R/dig.R to handle call_function == "dig_itemsets" and call dig_itemsets_().
  • Added new exported C++ entrypoint dig_itemsets_ in src/dig.cpp.
  • Wired generated interface layers in R/RcppExports.R and src/RcppExports.cpp.

• C++ storage specialization for itemsets

  • Added src/dig/ItemsetStorage.h, modeled after AssocStorage, but storing itemsets as:
    • items (formatted set)
    • support
    • count
    • length
  • Added DigItemsetRunner in src/dig.cpp to dispatch over the same chain-type strategy as other dig variants.

• Tests and package docs index updates

  • Added tests/testthat/test-dig_itemsets.R with basic output, max_results limiting, and argument validation coverage.
  • Exported symbol in NAMESPACE.
  • Added dig_itemsets to _pkgdown.yml reference index.

Example usage:

d <- partition(mtcars, .breaks = 2)

res <- dig_itemsets(
    d,
    items = everything(),
    min_support = 0.3
)

[codecov]

Codecov Report

❌ Patch coverage is 84.37500% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.75%. Comparing base (e7acebd) to head (82ce857).
⚠️ Report is 2 commits behind head on devel.

Files with missing lines	Patch %	Lines
src/dig/ItemsetStorage.h	65.71%	24 Missing ⚠️
R/dig_itemsets.R	98.64%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel      #54      +/-   ##
==========================================
- Coverage   86.86%   86.75%   -0.11%     
==========================================
  Files         127      129       +2     
  Lines        6516     6676     +160     
==========================================
+ Hits         5660     5792     +132     
- Misses        856      884      +28     

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