[codex] Add Groovy formula DSL docs and verification close-out

[perNyfelt]

Summary

Finish the Groovy formula DSL work for matrix-stats by landing the section 4 fit helpers, the DSL follow-up fixes, the section 5 documentation updates, and the final section 6 verification record.

What Changed

• Added FitDsl convenience entry points for lm, loess, and gam
• Added regression tests for FitDsl parity against FitRegistry.instance().get(...).fit(ModelFrame.of(...).evaluate())
• Centralized invalid-response | handling in TermExpr so non-variable response shapes report the formula-specific error instead of Groovy's generic missing-method error
• Removed redundant or(...) overrides from formula DSL subclasses once the base implementation was in place
• Documented the Groovy formula DSL in matrix-stats/README.md, docs/tutorial/3-matrix-stats.md, and docs/cookbook/matrix-stats.md
• Added the Groovy formula DSL and fit helper entries to matrix-stats/release.md
• Updated matrix-stats/req/v2.4.0-groovyFormulas.md through section 6 with the recorded validation commands

Why

The Groovy formula work needed three things to be complete:

1. Direct fit helpers so callers can write lm(data) { ... } without manually building a ModelFrame
2. Consistent DSL failure messages for invalid response-side expressions
3. Documentation and recorded verification so the 2.4.0 feature scope is complete and reviewable

User Impact

Callers can now write:

lm(data) { y | x + group }
loess(data) { y | x }
gam(data) { y | smooth(time, 6) + group }

The docs now also describe:

• string-vs-Groovy formula parity
• deferred syntax (| and noIntercept)
• dynamic propertyMissing / operator-dispatch behavior
• the supported arithmetic scope inside I { ... }
• interaction(x, group, z) for three-way interactions

Validation

• GRADLE_USER_HOME=$PWD/.gradle-user ./gradlew :matrix-stats:test --tests 'formula.GroovyFormulaDslTest'
• GRADLE_USER_HOME=$PWD/.gradle-user ./gradlew :matrix-stats:test --tests 'formula.*' --tests 'regression.*'
• GRADLE_USER_HOME=$PWD/.gradle-user ./gradlew :matrix-stats:spotlessApply :matrix-stats:test :matrix-stats:codenarcMain :matrix-stats:codenarcTest
• git diff --check -- matrix-stats/README.md docs/tutorial/3-matrix-stats.md docs/cookbook/matrix-stats.md matrix-stats/release.md
• GRADLE_USER_HOME=$PWD/.gradle-user ./gradlew test

All of the above passed. The final full-repo run completed with:

BUILD SUCCESSFUL in 17m 6s
530 actionable tasks: 529 executed, 1 up-to-date
Configuration cache entry stored.