fix(sleep): keep train and val splits non-empty

[kaluli123123]

Summary

• Preserve hash-assigned test tasks whenever the non-test pool can supply distinct train and validation tasks.
• When a small batch cannot supply both pools, deterministically reassign the minimum number of test tasks and emit a warning with each affected task ID and destination split.
• Cover both the reported all-test case and the adjacent val-plus-test/no-train case so the fallback cannot empty validation while creating train.

Why

assign_splits promised a non-empty validation gate for at least two real tasks, but an all-test hash assignment left both train and validation empty. A val-plus-test assignment had the same invariant gap: topping up train from the only validation task emptied validation.

The fallback now prefers spare train or validation tasks and reaches into test only when preserving both required pools is otherwise impossible.

Fixes #271

Validation

• uv run --extra dev pytest -q tests/test_split_hardening_2x3.py tests/test_split_wiring.py — 32 passed.
• Exhaustive local matrix over 6,300 combinations of seed, task count, split fractions, and input order — all invariants passed.
• PATH=/Users/vam/.local/bin:/usr/bin:/bin:/usr/sbin:/sbin uv run --extra dev pytest -q — 1,499 passed, 11 skipped, 353 subtests passed.
• uv run --extra dev ruff check skillopt_sleep/mine.py tests/test_split_hardening_2x3.py — passed.
• git diff --check — passed.

Baseline notes

With the normal host PATH, the full suite reports one unrelated failure because the installed Pi CLI resolves to /opt/homebrew/bin/pi while tests/test_backend_pi.py::test_default_model_from_env expects the literal string pi. The same failure reproduces on pristine upstream main; excluding the installed Pi CLI from PATH gives the clean full-suite result above. Full-repository Ruff also reports pre-existing lint findings outside these two files; both changed files are clean.

[kaluli123123]

@microsoft-github-policy-service agree