ERR/E4.1 (slice 1): log + is_comptime + process.exit/assert (+ noreturn codegen)

Stdlib slice of Phase E4, plus the noreturn codegen fix that enables it.

noreturn codegen (the enabling bug): E1.4c made `noreturn` type-system-only;
this is its first backend consumer and it crashed LLVM verification. Fixed:
- lower.zig: a `-> noreturn` body lowers as statements ending in `unreachable`
  (ensureTerminator emits unreachable; the two body-lowering sites no longer
  treat the last expr as a `ret`).
- emit_llvm.zig: a `void`/`noreturn` call result stays unnamed (direct +
  foreign call sites) — LLVM rejects a named void value.
- finishCatchHandler: a `noreturn` value-carrying catch body (which is not an
  IR terminator) closes the handler with `unreachable` instead of feeding a
  bad value into the merge phi. Shared by lowerCatch + lowerCatchOverChain.

is_comptime(): new nullary `.is_comptime` IR op (inst/print/interp/emit_llvm) —
interp evaluates true, emit_llvm emits constant false, so `if is_comptime()`
dead-codes out of compiled binaries. Recognized by name in
tryLowerReflectionCall + inferExprType (no std.sx decl, which would emit a
spurious `declare @is_comptime` into every module).

library/modules/log.sx: warn/info/debug/err — interpolate like print, write
`LEVEL: <msg>` to stderr. (`error` is reserved → the level is `log.err`.)

process.exit(code) -> noreturn + assert(cond, msg) in process.sx. `exit` is
POSIX `_exit(2)` (immediate, no cleanup; sx print is unbuffered so nothing is
lost), bound to "_exit" which also avoids a link-level clash with the sx `exit`
function's own name.

examples 248 (exit 0), 249 (exit 42), 250 (exit 1). #caller_location, the
comptime-exit diagnostic flush, and trace.print_interpreter_frames deferred to
E4.1b.

src/ir/interp.zig

@@ -599,6 +599,7 @@ pub const Interpreter = struct {
             .const_string => |sid| return .{ .value = .{ .string = self.module.types.getString(sid) } },
             .const_null => return .{ .value = .null_val },
             .const_undef => return .{ .value = .undef },
+            .is_comptime => return .{ .value = .{ .boolean = true } },
             .const_type => |tid| return .{ .value = .{ .type_tag = tid } },
 
             // ── Arithmetic ──────────────────────────────────────