fix(emit-lib): weak aether_lib_meta so N lib TUs link with no dup flags

CHANGELOG.md

@@ -9,6 +9,22 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 `main`, the release pipeline automatically replaces `[current]` with the next
 version number before tagging the release.
 
+## [current]
+
+### Fixed
+- `--emit=lib`'s `aether_lib_meta()` catalog entry point is now emitted
+  weak (`__attribute__((weak))` under GCC/Clang), so linking N lib TUs
+  into one artifact — aeb's regen shape, one `--emit=lib` C per module
+  and one final link — no longer dies with N-1 "multiple definition of
+  `aether_lib_meta`" errors or forces `-Wl,--allow-multiple-definition`
+  back onto the link line (the GNU-only escape hatch the 0.539 multi-TU
+  static-clone model retired; ld64 rejects it) (#1590). Lone-`.so`
+  consumers are unchanged: one definition, same dlsym contract; in a
+  multi-TU link the first TU's catalog wins. The
+  `multi_tu_import_link` regression now has an `--emit=lib` phase that
+  links three catalog-bearing TUs with no escape hatch and pins the
+  weak linkage via nm.
+
 ## [0.540.0]
 
 ### Added

docs/emit-lib.md

@@ -534,6 +534,15 @@ source file, source line, in a layout-stable, allocation-free form
 that any FFI consumer (Python ctypes, Java Panama, Ruby Fiddle,
 Node-API, hand-rolled `dlsym`) can walk directly.
 
+The entry point is emitted **weak** (`__attribute__((weak))` under
+GCC/Clang, #1590), so an orchestrator that links several `--emit=lib`
+TUs into one artifact (aeb's regen shape: one C per module, one final
+link) needs no `-Wl,--allow-multiple-definition` — the multi-TU
+static-clone link model of 0.539 holds for lib TUs too, on ld64 as well
+as GNU ld. In a lone `.so` nothing changes: one definition, same dlsym
+contract. In a deliberate multi-TU link the first TU's catalog wins;
+treat `aether_lib_meta` as meaningful only on single-module artifacts.
+
 ```c
 typedef struct {
     const char* aether_name;     /* "double_int", "std.fs.copy"      */

tests/integration/multi_tu_import_link/test_multi_tu_import_link.sh

@@ -60,4 +60,62 @@ if command -v nm >/dev/null 2>&1; then
     fi
 fi
 
-echo "  [PASS] multi_tu_import_link: 4 TUs, no dedup flags, runs (42 43)"
+# ---- Phase 2: the SAME shape with --emit=lib TUs (#1590) ------------------
+# aeb regen TUs are emitted with --emit=lib, and every lib TU carries the
+# aether_lib_meta() reflection catalog (issue #403). Before #1590 that
+# symbol had external linkage, so this exact link died with N-1
+# "multiple definition of `aether_lib_meta'" errors — a hole the plain-
+# emission phase above cannot see. The catalog entry point is now weak.
+TMP2="$TMP/libtu"
+mkdir -p "$TMP2"
+for m in helper/module a/module b/module; do
+    out="$TMP2/$(echo "$m" | tr '/' '_').c"
+    ( cd "$SCRIPT_DIR" && "$AETHERC" --emit=lib "$m.ae" "$out" ) >"$TMP2/cc.log" 2>&1 \
+        || fail "aetherc --emit=lib failed on $m.ae" "$TMP2/cc.log"
+done
+# The driver stays plain emission (it has main()).
+( cd "$SCRIPT_DIR" && "$AETHERC" main.ae "$TMP2/main.c" ) >"$TMP2/cc.log" 2>&1 \
+    || fail "aetherc failed on main.ae (lib phase)" "$TMP2/cc.log"
+
+for c in "$TMP2"/*.c; do
+    $CC -c "$c" -o "${c%.c}.o" $CFLAGS_ALL 2>"$TMP2/gcc.log" \
+        || fail "lib-TU compile failed: $c" "$TMP2/gcc.log"
+done
+
+# Load-bearing: three catalogs, one link, still NO escape hatch.
+if ! $CC "$TMP2"/*.o -o "$TMP2/repro_lib" $LIBS_ALL 2>"$TMP2/link.log"; then
+    if grep -qi "aether_lib_meta" "$TMP2/link.log"; then
+        fail "duplicate aether_lib_meta across --emit=lib TUs (#1590 regression)" "$TMP2/link.log"
+    fi
+    if grep -qiE "duplicate symbol|multiple definition" "$TMP2/link.log"; then
+        fail "duplicate symbols across --emit=lib TUs" "$TMP2/link.log"
+    fi
+    fail "lib-TU link failed" "$TMP2/link.log"
+fi
+
+out=$("$TMP2/repro_lib" 2>&1)
+[ "$out" = "42 43" ] || fail "wrong output from lib-TU build: '$out'"
+
+# Belt: the catalog entry point is emitted weak, not strong. The nm
+# spelling is format-specific: ELF/COFF nm marks a weak definition
+# "W"/"w", but Mach-O nm prints it as a plain "T" and only `nm -m`
+# reveals the "weak external" flag — grepping for T there would
+# false-fail the exact platform this model exists for (ld64).
+if command -v nm >/dev/null 2>&1; then
+    a_lib_o=$(ls "$TMP2"/a_module.o)
+    case "$(uname -s)" in
+    Darwin)
+        nm -m "$a_lib_o" | grep -q "weak external _aether_lib_meta" \
+            || fail "aether_lib_meta missing/not weak in a lib TU (nm -m)"
+        ;;
+    *)
+        if nm "$a_lib_o" | grep -qE " T _?aether_lib_meta"; then
+            fail "aether_lib_meta has strong external linkage in a lib TU (#1590)"
+        fi
+        nm "$a_lib_o" | grep -qE " [Ww] _?aether_lib_meta" \
+            || fail "aether_lib_meta missing/not weak in a lib TU"
+        ;;
+    esac
+fi
+
+echo "  [PASS] multi_tu_import_link: 4 TUs, no dedup flags, runs (42 43); --emit=lib TUs link too (#1590)"