Add tests for build pipeline scripts (build_v4_snapshot.py, validate_v4_dataset.py)

[shaypal5]

Problem

scripts/build_v4_snapshot.py (213 lines) and scripts/validate_v4_dataset.py (334 lines) have no automated tests. The validation script is itself a form of integration test for the generated CSV, but individual functions like subsample(), inject_missingness(), and the validation checks themselves are untested in isolation.

Suggested scope

• Unit tests for subsample() edge cases (insufficient positives/negatives)
• Property tests for inject_missingness() (rates within expected bounds)
• Unit tests for each validation check function
• Integration test: build_v4_dataset() → validate() returns 0

These scripts live in scripts/ (not leadforge/), so they need explicit test discovery or a test helper that imports them.

Context

Identified in self-review of PR #21. The scripts will stabilize further in v4-M2 (release), making that a natural point to add tests.