Switch on String: forward default into each length bucket

[Pign]

Summary

compileSwitchOptimizedForStrings (in src/cxxcompiler/subcompilers/Expressions.hx) buckets cases by String.length and emits a switch(__temp.size()) { case N: { /* if-cascade */ } }. It calls compileSwitchAsIfs per bucket with null for the default expression, so the user's default: only ends up attached to the outer size-switch via compileDefaultCase(edef). That fires when the input's length matches no bucket, but not when the length matches a bucket and the value matches no case inside it. Such inputs fall through the inner if-cascade with no statement executed — silently breaking statement-position string switches and tripping -Werror=return-type on value-position ones.

Repro

switch (ch) {           // ch is a 1-char String
    case "H": ...
    case "W": ...
    case "d": ...
    case "e": ...
    default:  return 0;  // never fires for unknown 1-char inputs
}

The generated C++ has case 1: { if (__temp == "H") { ... } else if (__temp == "W") ... } with no terminating else, so an input like "X" exits via break;, the size-switch finishes, and the function returns nothing.

(Caught while writing a small bitmap-font helper in a real Haxe→C++ project; the -Werror=return-type from gcc/clang made it surface immediately.)

Fix

Pass edef into the bucketed compileSwitchAsIfs so each cascade ends with else { /* user default */ }. The outer-default emission is kept — it still covers unmatched-length inputs.

Test plan

• Two regression tests added in test/unit_testing/tests/Switch/:
  • statement-position switch (matches the bucket length, falls through to default)
  • value-position switch (must produce a value for unmatched same-length inputs, otherwise the C++ output trips -Werror=return-type)
• Each is wrapped in its own helper function because the codegen declares auto __temp = ...; at function scope; calling two string switches from the same function would currently collide on __temp. That's a separate, pre-existing issue and isn't in scope here.
• All 46 existing unit tests still pass (haxe Test.hxml).
• Generated C++ for the new tests builds clean under -Werror.

Reviewed in our fork at Pign#1 before opening here.