P5.7 Step A: VM is the sole comptime evaluator at emit-time + type-fn sites (no fallback)

Remove the comptime_flat/need_vm gate and the vm_result-orelse-legacy
fallback from emit_llvm.zig (runComptimeSideEffects + emitGlobals const-init)
and comptime.zig (runComptimeTypeFunc). The comptime VM now always runs;
a bail is always a build-gating diagnostic, never a fallback. Delete the
now-moot entryNeedsVm. runComptimeSideEffects drops the Interpreter entirely
(VM writes #run output direct to fd 1); emitGlobals keeps a fresh interp_inst
only as the valueToLLVMConst materialization context (the regToValue bridge,
removed with interp.zig in a later step).

#insert (evalComptimeString) still routes through the legacy interp — deferred
until interp.zig deletion.

Reconcile 1654: the comptime asm-global #run now reports the VM's clean dlsym
bail instead of the legacy CannotEvalComptime wrapper (exit still 1).

501/501 unit + 706/0 corpus.

examples/expected/1654-platform-asm-global-comptime-call.stderr

@@ -1 +1 @@
-error: comptime init of 'COMPUTED' failed: CannotEvalComptime (op=call: comptime extern call: symbol not found via dlsym (target-specific binding called at compile time?))
+error: comptime init of 'COMPUTED' failed: comptime extern call: symbol not found via dlsym (target-specific binding called at compile time?)