fix(decimal): reserve integer digit budget #18

Summary

• Reserve whole-number digits according to precision - scale for decimal checks.
• Capture measured precision and scale with the failure decision for consistent message arguments.
• Cover boundaries, signs, zero, trailing zeroes, and invalid settings.
• Update the source mirror and public API description.

Closes #17.

Validation

• rg --files src test -g '*.pudu' | xargs pudu check
• pudu lint src test
• pudu fmt --check src test
• pudu test test — 18 suites, 163 assertions
• pudu build src/Main.pudu -o /tmp/pudu-lang-validator
• Compiled CLI --version smoke run
• Independent source and wiki review