Add Phase 4: code quality, CI/CD, and formatting

- testthat infrastructure with 15 tests covering env-var guards,
  return types for all format/save functions, and spelling
- inst/WORDLIST with 52 domain terms (LightGBM, MinIO, Parquet, etc.)
- Spelling test wired into devtools::test() via test-spelling.R
- styler::style_file() added as step 0 in deploy.R (auto-fixes before ship)
- .gitea/workflows/test.yaml: runs testthat suite on push
- .gitea/workflows/lint.yaml: lychee link check + styler dry-run on push
- Removed internal IP address from comment in train_production_model()
- Language: en-US added to DESCRIPTION

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

inst/WORDLIST

@@ -0,0 +1,52 @@
+Acknowledgements
+Adasyn
+ADASYN
+anonymized
+baf
+BAF
+colorspace
+conf
+CTGAN
+datasheet
+DuckDB
+EDA
+env
+FN
+FP
+FPR
+frac
+ggplot
+Gu
+Guo
+Hexbin
+Kaggle
+lakehouse
+Lakehouse
+lgbm
+LightGBM
+LightGBM's
+MinIO
+NeurIPS
+optimise
+Optimises
+pos
+pre
+qmd
+rds
+relabelled
+Renviron
+revealjs
+RevealJS
+Scalability
+serialised
+Shang
+Sig
+tabset
+tbl
+tibble
+Tibble
+tidymodels
+Tomek
+TP
+Undersampling
+XGBoost