test: add unit tests for configs, base utilities, and torch layers

[ademakdogan]

Summary

Add a comprehensive test suite for TimesFM v2.5 components that previously had no test coverage. All 58 tests run on CPU without requiring a model checkpoint and complete in under 1 second.

Motivation

The current test suite (v1/tests/test_timesfm.py) only covers the archived v1 API. The v2.5 source code under src/timesfm/ — including configs, NaN-handling utilities, running statistics, RevIN normalization, and layer building blocks — has zero test coverage. These functions sit on the critical inference path; silent regressions here would manifest as degraded forecast quality rather than immediate errors.

What's Tested

tests/test_configs.py (10 tests)

• ForecastConfig frozen immutability and default value regression guard
• dataclasses.replace independence (used in the compile path)
• Structural equality semantics
• ResidualBlockConfig, TransformerConfig, StackedTransformersConfig

tests/test_base_utils.py (16 tests)

• strip_leading_nans: edge cases including all-NaN behavior
• linear_interpolation: interior/leading/trailing NaN filling, monotonicity preservation, single-value extrapolation, output NaN-freedom guarantee

tests/test_torch_utils.py (14 tests)

• update_running_stats: mathematical correctness verified against NumPy, incremental accumulation equivalence, masked element exclusion, batch independence, constant-input zero-sigma
• revin: forward/reverse identity invariant, zero-sigma NaN guard, tolerance threshold, 3D and 4D broadcasting matching actual decode shapes
• DecodeCache: mutability contract, key/value shape consistency

tests/test_torch_layers.py (18 tests)

• ResidualBlock: output shapes (2D/3D), residual path isolation, gradient flow through both paths, all activations, invalid activation rejection
• RMSNorm: zero-scale initialization property, unit-scale RMS magnitude, zero-input NaN safety
• RandomFourierFeatures: output shape, dim % 4 constraint, numerical bounds

Quality Checks

• ruff check tests/ — All checks passed
• ruff format --check tests/ — All files formatted
• pytest tests/ -v — 58 passed in <1s
• No model checkpoint or GPU required
• Apache 2.0 license headers matching existing source files
• 2-space indent per pyproject.toml ruff config