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: this is a compile-time hint to silence GCC 16 -Wmaybe-uninitialized warnings, with no intended runtime behavior change. The main risk is undefined behavior if the switch becomes non-exhaustive in the future and hits __builtin_unreachable().

Overview
Fixes new GCC 16 -Wmaybe-uninitialized warnings in implementation_chooser.h by making the switch statements in CHOOSE_IMPLEMENTATION and CHOOSE_SVE_IMPLEMENTATION explicitly exhaustive.

Adds default: __builtin_unreachable() so the compiler can prove __ret_dist_func is always initialized before assignment, without changing the selected implementation for valid inputs.

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[CLAassistant]

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[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.92%. Comparing base (4ca500a) to head (17073f7).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #966      +/-   ##
==========================================
+ Coverage   96.71%   96.92%   +0.21%     
==========================================
  Files         129      130       +1     
  Lines        8057     7749     -308     
==========================================
- Hits         7792     7511     -281     
+ Misses        265      238      -27     

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