perf: speed up area-weighted interpolation

[dshkol]

Summary

Speeds up tongfen_estimate() while preserving its existing API and output semantics.

Changes

• Remove unnecessary casting of every intersection geometry to MULTIPOLYGON.
• Calculate source geometry areas once, then index them for each intersection.
• Scale all variables in one batch instead of repeatedly calling mutate().
• Replace deprecated tidyselect usage encountered on this path.
• Add tests covering:
  • Multiple additive variables
  • Geometry preservation
  • na.rm = TRUE and na.rm = FALSE

Performance

Benchmark fixture:

• 2,500 source polygons
• 2,500 offset target polygons
• 40 additive variables
• Seven timed iterations after warm-up
• R 4.5.0, sf 1.0.23, dplyr 1.1.4

Revision	Median	Range
v0.3.8	0.667s	0.651–0.711s
Candidate	0.207s	0.195–0.216s

Speedup: 3.22×

Complete outputs from separately installed baseline and candidate packages were equivalent using:

all.equal(
baseline,
candidate,
check.attributes = TRUE,
tolerance = 1e-12
)

Validation

• Full test suite passes.
• Source package builds successfully with vignettes.
• R CMD check --no-manual: Status OK
• No new dependencies.
• No public API changes.