feat: report area, point density and pulse density in summarise()

[agrigoriev]

Summary

Ports lidR's scalar density() quality-control check into lasR. The
summary stage now computes the coverage area in a single streaming pass
and exposes three new fields in the summarise() output list:

• area — processed bounding-box area (sum of per-chunk core extents,
  buffer excluded)
• density — point density (npoints / area)
• pulse_density — pulse density (first returns / area)

u <- exec(reader() + summarise(), on = f)
u$area; u$density; u$pulse_density

Semantics

• Area uses the sum of per-chunk core (buffer-excluded) extents, matching
  lidR's density(LAScatalog) (sum of per-file bounding boxes).
• Following lidR's print/summary convention, the number of pulses is the
  count of first returns (ReturnNumber == 1), already collected by the
  stage as npoints_per_return[1]. It is not derived from
  gpstime-based pulse identification (retrieve_pulses).
• Density is buffer-independent: buffer points are excluded from both
  npoints and area.

Implementation

• summary.h — double area member + set_chunk override.
• summary.cpp — accumulate core-extent bbox area per chunk in
  set_chunk; sum in merge; emit area, density, pulse_density in
  both to_R() and to_json().
• R/stages.R + man/summarise.Rd — document the new fields and the
  "pulses = first returns" convention.

Tests

Added to tests/testthat/test-summarise.R:

• single-file relational check (density == npoints/area,
  pulse_density == npoints_per_return["1"]/area)
• multi-file (bcts) check that pulse density uses the first-return count

Verified bbox bcts values: 13.7 points/m², 9.6 pulses/m².

[codecov]

Codecov Report

❌ Patch coverage is 84.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.27%. Comparing base (6ec1b89) to head (cec3307).
⚠️ Report is 22 commits behind head on devel.

Files with missing lines	Patch %	Lines
src/LASRstages/summary.cpp	84.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #326      +/-   ##
==========================================
+ Coverage   72.15%   72.27%   +0.11%     
==========================================
  Files         230      230              
  Lines       17576    17657      +81     
==========================================
+ Hits        12682    12761      +79     
- Misses       4894     4896       +2     

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