feat: implement Stand Density Index (SDI) autoselector and classification

[aitorvv]

Description

This PR implements the automated selector for Reineke's Stand Density Index (SDI) and the maximum stand density classification, matching the library patterns established in the biomass predictors.

Key Changes:

• silv_density_sdi_auto(): Dynamically retrieves the correct beta exponent from the sdi_coefficients dataset using a cascading fallback system (exact match -> country -> genus -> default Reineke -1.605).
• Automated SDImax & Model Info: Integrates maximum density estimates via the internal sdimax_models dataset. The output data.frame now always includes the computed sdimax and the string descriptor sdimax_model (e.g. rodriguez-prado-2020 (Spain, basic)).
• Classification (classify = TRUE): Enables qualitative density classes based on the relative density thresholds.
• _pkgdown.yml: Registered silv_density_sdimax and sdimax_models in the documentation references.
• Tests: Expanded suite test-stand-density.R to cover the fallback mechanisms, models, and outputs.

Verification

You can copy and paste the following R code to verify the changes:

devtools::load_all()

# 1. Verify datasets
str(sdi_coefficients)
str(sdimax_models)

# 2. Verify SDI Autoselector & Fallbacks
res1 <- silv_density_sdi_auto(
  ntrees = 800,
  dg = 23.4,
  species = "Pinus sylvestris",
  country = "Spain",
  region = "Castilla y León"
)
# Expected: beta = -1.75, sdi_model = "del-rio-2006 (Spain, Castilla y León)", sdimax_model = "rodriguez-prado-2020 (Spain, basic)"
print(res1)

# 3. Verify SDI Autoselector with fallback to Reineke default
res2 <- silv_density_sdi_auto(
  ntrees = 800,
  dg = 23.4,
  species = "Unknown species"
)
# Expected: beta = -1.605, sdi_model = "reineke-1933 (-1.605)"
print(res2)

# 4. Verify Classification and Climate models
res3 <- silv_density_sdi_auto(
  ntrees = 800,
  dg = 23.4,
  species = "Pinus canariensis",
  classify = TRUE,
  climatic_model = "P1",
  clim_value = 400
)
# Expected: sdimax_model = "rodriguez-prado-2020 (Spain, P1)", sdi_class should be computed.
print(res3)