ffi issue-0047: file #run-on-stderr-vs-runtime-on-stdout split

Surfaced while adding the `--- build done ---` delimiter
(commit 2993072). `#run print()` output is buffered by the
interp and flushed via std.debug.print → stderr at
core.zig:187/190; JIT runtime `print()` writes via libc
write(1, ...) → stdout. Same `print` call from the user's
viewpoint, different streams in practice.

Not blocking step 3 — tests capture both streams via 2>&1 so
snapshots are unaffected. Issue file documents the fix path
(move the two `std.debug.print` flushes in core.zig to
stdout-writes) for a future session.

issues/0047-run-output-on-stderr-runtime-on-stdout.md

@@ -0,0 +1,108 @@
+# Symptom
+
+`#run print(...)` output lands on **stderr**; runtime `print(...)`
+output lands on **stdout**. The test runner captures both via
+`2>&1` so they appear interleaved in snapshots, but for a human
+running `sx run foo.sx` and piping stdout somewhere — or in any
+context that separates the streams — the compile-time output
+silently goes to a different place than the runtime output.
+
+The recently-landed `--- build done ---` delimiter (commit
+`2993072`) makes the boundary visible in test logs but doesn't
+fix the underlying split.
+
+# Reproduction
+
+```sx
+#import "modules/std.sx";
+
+configure :: () {
+    print("hello from #run\n");
+}
+#run configure();
+
+main :: () -> s32 {
+    print("hello from runtime\n");
+    return 0;
+}
+```
+
+```
+$ ./zig-out/bin/sx run repro.sx > /tmp/stdout.txt 2> /tmp/stderr.txt
+$ cat /tmp/stdout.txt
+hello from runtime
+$ cat /tmp/stderr.txt
+hello from #run
+```
+
+Expected (most consistent): both on stdout — both are `print`
+calls in user code; the user doesn't distinguish "build-time
+print" from "runtime print" at the call site.
+
+# Root cause
+
+The interp accumulates `print` output into an internal buffer
+(`Interpreter.output: []u8`) via the `out` builtin
+(`src/ir/interp.zig:1678-1683`). After the interp returns, the
+buffer is flushed to **stderr** via `std.debug.print` from
+`src/core.zig:187` and `src/core.zig:190`.
+
+The JIT-executed runtime `print` lowers through `BuiltinId.out`
+in `emit_llvm.zig:2936-2954` which emits `write(1, ptr, len)` —
+directly to fd 1 (stdout).
+
+So the split is at the flush call in `core.zig`, not at the
+print mechanism itself.
+
+# Investigation prompt
+
+For a fresh session picking this up:
+
+The fix is small in code but touches a few callers. The two
+`std.debug.print("{s}", .{interp.output.items})` sites at
+`src/core.zig:187` and `:190` should write to stdout instead.
+
+Options for the stdout write:
+1. **libc write(1, ...)** via `std.c.write(1, ptr, len)`. Direct,
+   no Zig std buffering, interleaves correctly with `--- build
+   done ---` (also direct to stderr).
+2. **std.posix.write(std.posix.STDOUT_FILENO, ...)** — Zig'\''s
+   typed wrapper. Same effect.
+3. **std.fs.File.stdout() + writeAll** — more idiomatic but
+   may bring buffering complexity.
+
+Whichever route, the order of writes vs the `--- build done ---`
+delimiter (which currently goes to stderr) matters:
+- If both #run output AND delimiter both go to stdout: ordering
+  preserved within stdout's buffer.
+- If delimiter stays on stderr and #run goes to stdout: the
+  delimiter might appear out-of-order in mixed-stream captures.
+
+Cleanest: move BOTH to stdout. The compile-error path (in
+`renderErrors`) stays on stderr — only successful #run output
+moves.
+
+Other call sites that might leak `#run` output to stderr:
+- `core.zig:187` — `invokeByFuncId` error path.
+- `core.zig:190` — `invokeByFuncId` success path.
+- Grep for `interp.output` to find others.
+
+# Verification
+
+After the fix, the repro above should produce:
+
+```
+$ ./zig-out/bin/sx run repro.sx > /tmp/stdout.txt 2> /tmp/stderr.txt
+$ cat /tmp/stdout.txt
+hello from #run
+--- build done ---
+hello from runtime
+$ cat /tmp/stderr.txt
+(empty)
+```
+
+Test snapshots for the 7 tests with top-level `#run` will need
+to be re-`--update`d, but the visible content is the same —
+just on the right stream now.
+
+Full suite + `zig build test` must still pass.