Fix -Wmaybe-uninitialized with GCC 16 in CHOOSE_IMPLEMENTATION macros

[daxhuiberts]

GCC 16 improved its uninitialized variable analysis and now warns that __ret_dist_func may be used uninitialized: the switch is exhaustive (e.g. CASES16 covers exactly 0-15 and dim % 16 can only produce 0-15), but GCC cannot prove this from the recursive macro expansion.

Add default: __builtin_unreachable() to both CHOOSE_IMPLEMENTATION and CHOOSE_SVE_IMPLEMENTATION to explicitly signal to the compiler that no other case is reachable, allowing it to conclude the variable is always initialized before use.

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Note

Low Risk
Low risk, compile-time-only change adding default: __builtin_unreachable() to exhaustive switch macros to satisfy newer GCC uninitialized-variable analysis. Runtime behavior should be unchanged unless invariants are violated, in which case it becomes UB as intended.

Overview
Fixes GCC 16 -Wmaybe-uninitialized warnings in the SIMD implementation selection macros by making the switch statements explicitly exhaustive.

Adds default: __builtin_unreachable() to both CHOOSE_IMPLEMENTATION and CHOOSE_SVE_IMPLEMENTATION, ensuring __ret_dist_func is always considered initialized when the remainder/step calculations are within expected ranges.

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