Replace JNI dtype maps with constexpr functions

[cyyever]

The two const unordered_maps in jni_layer_constants.h were static-initialized and heap-allocated in every including TU; replace them with constexpr switch functions (round-trip verified via static_assert). Also make Stats::SCALING_FACTOR_UNITS_PER_SECOND static constexpr so it no longer occupies space in every instance and stops blocking copy-assignment.

Authored with Claude.

[pytorch-bot]

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