Wire val_size through CLI and pipeline construction

[Copilot]

Description

Daily audit surfaced a split-configuration gap: val_size existed in config/pipeline logic but could not be set via CLI/Makefile, so runs silently defaulted to 0.1. This PR makes validation split sizing externally configurable and consistently propagated.

Changes in this pull request

• CLI/config wiring

  • Added --val-size to argument parsing in src/main.py.
  • Mapped parsed value into HyperionConfig(val_size=...).

• Pipeline propagation

  • Passed config.val_size into both:
    • StackedModelTrainingPipeline(...)
    • TimeSeriesStackedModelTrainingPipeline(...)

• Runtime entrypoint parity

  • Added VAL_SIZE ?= 0.1 in Makefile.
  • Included --val-size $(VAL_SIZE) in _RUN_ARGS.

• Docs alignment

  • Updated README run examples and parameter table to include VAL_SIZE / --val-size.

• Focused regression coverage

  • Added tests/test_main_parse_args.py for:
    • default val_size parsing
    • explicit --val-size override

parser.add_argument(
    "--val-size",
    type=float,
    default=defaults.val_size,
    help="Fraction of training data reserved for validation (0 < val_size < 1 - test_size).",
)