ERR/E4.2: failable-main wrapper (report + exit 1 on escaping error)

A pure-failable `main` (`-> !` / `-> !Named`) that lets an error reach the
function boundary now exits 1 and prints `error: unhandled error reached
main: error.<tag>` + the return trace to stderr, instead of returning the
raw tag id truncated as the exit code with no diagnostic. Success exits 0;
a `catch`-absorbed error exits 0 (buffer cleared).

Codegen wrapper so JIT and AOT behave identically (no host-side special-
casing):
- emit_llvm.zig: the `.ret` arm detects a failable main and routes to
  new `emitFailableMainRet` — `icmp ne tag, 0` → success block `ret i32 0`
  / error block GEPs the tag name out of the always-linked tag-name table,
  calls `sx_trace_report_unhandled`, `ret i32 1`. main's bare-u32 returns
  (success `ret(0)` + each raise's `ret(tag)`) all funnel through it.
- sx_trace.c: new `sx_trace_report_unhandled(tag, name, name_len)` prints
  the header + surviving frames to stderr (placeholder frame format mirrors
  trace.sx until DWARF/E3.0). Lives next to the buffer it reads.
- lower.zig validateMainSignature: the pure-failable arm sets
  needs_trace_runtime so the AOT path auto-links sx_trace.c even when the
  body emits no other push/clear.

Value-carrying `-> (T, !)` main stays gate-rejected (multi-slot wrapper is
a separate slice). examples/244-failable-main.sx.

tests/expected/244-failable-main.exit

@@ -0,0 +1 @@
+1

tests/expected/244-failable-main.txt

@@ -0,0 +1,5 @@
+v = 10
+error: unhandled error reached main: error.Empty
+error return trace (most recent call last):
+  frame 0: <location pending DWARF> (raw 1)
+  frame 1: <location pending DWARF> (raw 1)