feat(summary): implement silv_tree_summary with integrated predictions, SNFI plots, and stand volume/biomass/carbon

[aitorvv]

Summary of Changes

• Implements silv_tree_summary():
  • Computes tree class (dclass), tree basal area (g), expansion factor (expan), expanded basal area (g_ha), tree slenderness, and competition indices (bal, bas).
  • Supports integrated on-the-fly predictions for volume (vcc, vsc, iavc), biomass (biomass), and carbon (carbon).
• Adds support for concentric Spanish National Forest Inventory plots (plot_shape = "snfi").
• Extends silv_summary() to calculate and aggregate stand volume (v_ha in $m^3/\text{ha}$), biomass (w_ha in $t/\text{ha}$), and carbon (c_ha in $t/\text{ha}$).
• Extends silv_summary_species() (V_sp1, W_sp1, C_sp1) and silv_summary_mortality() (V_alive/V_dead, W_alive/W_dead, C_alive/C_dead).
• Added unit tests in tests/testthat/test-tree-summary.R and tests/testthat/test-summary-thinning.R.
• Registered silv_tree_summary in _pkgdown.yml.

Note: This PR builds on top of avv-snfi-stand-functions.

Verification

devtools::load_all()

# 1. Verify tree summary with SNFI concentric rings and predictions
df_tree <- inventory_samples |> dplyr::filter(plot_id == 8) |> dplyr::mutate(province = 1)
res_tree <- silv_tree_summary(
  df_tree, diameter, height, plot_id, species, province = province,
  plot_shape = "snfi", compute_bal = TRUE,
  predict_volume = TRUE, predict_biomass = TRUE, predict_carbon = TRUE
)
stopifnot(all(c("dclass", "g", "expan", "g_ha", "slenderness", "bal", "bas", "vcc", "biomass", "carbon") %in% names(res_tree)))

# 2. Verify stand summary with volume (m3/ha), biomass (t/ha), carbon (t/ha)
res_stand <- silv_summary(
  df_tree |> dplyr::filter(species == 28), diameter, height,
  plot_shape = "snfi", .groups = "plot_id", species = species, province = province,
  predict_volume = TRUE, predict_biomass = TRUE, predict_carbon = TRUE
)
gm <- res_stand@group_metrics
stopifnot(all(c("ntrees_ha", "g_ha", "v_ha", "w_ha", "c_ha", "slenderness") %in% names(gm)))
stopifnot(gm$v_ha > 0, gm$w_ha > 0, gm$c_ha > 0)