feat(tree-metrics): implement SNFI tree-level metrics and competition indices

[aitorvv]

Summary of Changes

• Implements tree-level functions adapted from SNFI support scripts:
  • silv_tree_expansion_factor(): Dynamic expansion factors for fixed area or concentric SNFI plots (radii 5, 10, 15, 25 m).
  • silv_tree_bal() & silv_tree_bas(): Basal Area in Larger/Smaller trees competition indices per plot.
  • silv_tree_crown_ratio(): Tree crown ratio calculation.
  • silv_tree_coordinates(): Tree Cartesian coordinates from distance and azimuth.
  • silv_tree_circumference(), silv_tree_mean_dbh(), silv_tree_slenderness().
• Full Roxygen2 documentation with examples and registered in _pkgdown.yml.
• Added unit tests in tests/testthat/test-tree-level.R.

Note: This PR follows after PR #26.

Verification

devtools::load_all()

# 1. Verify dynamic SNFI expansion factors
dbh_vec <- c(10, 18, 30, 50)
exp_vec <- silv_tree_expansion_factor(type = "snfi", diameter = dbh_vec)
stopifnot(all.equal(round(exp_vec, 2), c(127.32, 31.83, 14.15, 5.09)))

# 2. Verify BAL and BAS competition indices
df_bal <- inventory_samples |> dplyr::filter(plot_id == 8)
bal_vals <- silv_tree_bal(df_bal, plot_id = plot_id, tree_id = tree_id, diameter = diameter, plot_size = 10)
stopifnot(is.numeric(bal_vals), length(bal_vals) == nrow(df_bal))