Restore legacy soil raster compatibility and prepare Pythia 2.3.1

[wpavan]

Summary

This change restores backward compatibility with legacy one-band GHR soil
rasters while retaining support for the newer two-band encoded format.

It also prepares the Pythia 2.3.1 distribution, documentation, automated
testing, and Sri Lanka example-data workflow.

User impact

Users of the existing Sri Lanka example no longer need to modify or convert
ggcmi_soils_2.tif. Pythia automatically detects the one-band format and
resolves its soil identifiers through GHR.db.

Main changes

• automatic one-band and two-band soil raster detection;
• legacy GHR lookup through GHR.db;
• validation of referenced .SOL profiles;
• raster bounds and CRS handling;
• commands to configure and validate released example data;
• reproducible example-data ZIP packager and SHA-256 checksum;
• updated README, conceptual guide, release guide, and release notes;
• corrected plugin hook context execution;
• updated GitHub release workflow;
• regression tests for the reported Sri Lanka coordinate.

Validation

• 37 tests passed;
• wheel and source distribution built successfully;
• real Sri Lanka archive validated;
• ZIP integrity and SHA-256 checksum verified;
• archive contains portable JSON templates and no personal paths;
• archive size: 498.4 MiB.