Make native NNS distance the default (dist=None) and propagate through stack/boost, add tests

[OVVO-Financial]

Motivation

• Expose the package's native blended NNS metric as the default distance when callers omit dist, preserving the historical / documented native behavior.
• Ensure the new default is available consistently across public APIs (nns_reg, nns_m_reg, nns_stack, nns_boost) and internal engines so ensemble and forecasting code use the intended metric.
• Make the metric explicit and testable while keeping the legacy explicit options ("L1", "L2", "FACTOR") supported.

Description

• Changed public signatures to accept dist: str | None = None for nns_reg, nns_m_reg, nns_stack, and nns_boost and updated internal dispatch to treat None as the native "NNS" alias via _validate_dist.
• Implemented the native NNS distance formula in _mreg_distances as sum(abs(z) + z**2, axis=2) and updated prediction/score paths to support dist == "NNS".
• Propagated validated dist values through nns_boost/nns_stack so learner trials and final stacking respect the requested metric, and updated other call sites to pass explicit dist where test determinism required it.
• Updated R tools and man pages to reflect dist = NULL mapping to the native "nns" alias and updated Python documentation to explain dist=None semantics.
• Added new invariants/unit tests (tests/invariants/test_native_distance_defaults.py) and updated parity tests to include explicit dist="L2" in parity scenarios where deterministic L2 behavior was intended.

Testing

• Added and ran the new invariant test tests/invariants/test_native_distance_defaults.py, which validates dist=None equivalence with dist="NNS", the native distance formula, and propagation through nns_stack/nns_boost; the test passed.
• Ran the modified parity tests exercising nns_reg, nns_m_reg, nns_stack, and nns_boost with explicit dist="L2" where required; parity tests passed.
• Ran the project's test suite (unit/invariants/parity) with the changes and observed all automated tests related to the distance default and affected pathways succeeding.

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Codex Task