Add -Dcomptime-flat-strict / env SX_COMPTIME_FLAT_STRICT (implies comptime_flat):
at all three comptime sites (type-fn in lower/comptime.zig, const-init + #run in
emit_llvm.zig) a VM bail becomes a build-gating error naming the reason INSTEAD of
falling back to legacy. Forces every comptime eval onto the VM so the complete gap
set is enumerable in one sweep; when the corpus is green under strict mode AND every
example matches legacy, interp.zig can be deleted.
Default behaviour unchanged (699/0 both default gates). Fixed a wiring bug: the
type-fn site's local comptime_flat didn't include the strict flag (every type-fn
falsely reported <unknown>); strict now implies flat there too.
Swept the gap list (19 strict bails): switch_br (5, + unmasks a []Type-across-call
silent-wrong in 0114), compiler_call (6, = the BuildOptions->abi(.zig) extern
compiler migration), out (2), type_name (1), global_addr (1), interp_print_frames
(1), 2 negative-test diagnostics (1179/1180), 1 dlsym (1654). Recorded as the
deletion checklist in CHECKPOINT-COMPILER-API.md.
dcb1392255