selfhost-verify: arregla el split OUT_DIR host/target de minicargo en build-stage0-musl

Al construir el cargo del stage0 musl-host, minicargo (con RUSTC_TARGET seteado,
aunque host==target==musl) corre los build-scripts bajo cargo-build/host/build_X/
pero el crate target hace include!/include_bytes!(OUT_DIR/..) apuntando a
cargo-build/build_X/ (vacío) -> mrustc aborta con signal 6. Rompió en libsqlite3-sys
(bindgen.rs) y en el binario cargo (man.tgz).

Fix: symlink_outdirs() enlaza cada dir target -> host (salidas bit-idénticas porque
host==target), aplicado en un bucle de reintento alrededor del build incremental de
cargo (los sys-crates tardíos -cargo/curl/openssl/libgit2-sys/libssh2-sys- corren sus
scripts al final, así que un solo pase no basta). Documentado como snag #4 en el README.

Verificado: stage0 musl-host rustc 1.90.0 + cargo 1.90.0 construyen y corren con
host=x86_64-unknown-linux-musl; desbloquea proc-macros nativos en run_rustc.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/rust-frontier/README.md

@@ -62,6 +62,18 @@ to bootstrap a usable rust there:
    can't be reassigned by the makefile, so the `-musl` suffix append is skipped and
    the build contaminates the gnu output dir. Let it default.
 
+4. **minicargo OUT_DIR host/target split** (only when building the musl stage0's
+   cargo). With `RUSTC_TARGET` set, minicargo runs each cargo build-script under
+   `cargo-build/host/build_X/`, but the *target* crate's `include!` /
+   `include_bytes!(concat!(env!("OUT_DIR"), "/.."))` resolves `OUT_DIR` to
+   `cargo-build/build_X/` (no `host/`, empty) → mrustc aborts with signal 6. Hit
+   `libsqlite3-sys` (`bindgen.rs`) and the `cargo` binary (`man.tgz`). The gnu
+   build never split (single triple → single dir). Since host==target the script
+   outputs are bit-identical, so `build-stage0-musl.sh` symlinks the target dir to
+   the host dir (`symlink_outdirs`). Late sys-crates (cargo, curl, openssl,
+   libgit2-sys, libssh2-sys) run their scripts near the end, so it's applied in a
+   retry loop around the incremental cargo build.
+
 ## Workflow
 
 ```sh

scripts/rust-frontier/build-stage0-musl.sh

@@ -28,11 +28,42 @@ run() {
       "$@"
 }
 
+# minicargo OUT_DIR host/target split fix.
+# With RUSTC_TARGET set (even when host==target==musl) minicargo runs each cargo
+# build-script under cargo-build/host/build_X/, but the *target* crate's
+# include!/include_bytes!(concat!(env!("OUT_DIR"),"/..")) resolves OUT_DIR to
+# cargo-build/build_X/ (no host/, empty) -> mrustc aborts with signal 6. Bit us on
+# libsqlite3-sys (bindgen.rs) and the cargo binary (man.tgz). host==target so the
+# script outputs are bit-identical; symlink target dir -> host dir. Late sys-crates
+# (cargo, curl, openssl, libgit2-sys, libssh2-sys) only run their scripts near the
+# end, so this is (re)applied between incremental cargo-build retries below.
+symlink_outdirs() {
+  cb="$MRUSTC/output-1.90.0-$T/cargo-build"
+  [ -d "$cb/host" ] || return 0
+  for d in "$cb"/host/build_*/; do
+    [ -d "$d" ] || continue
+    b=$(basename "$d")
+    [ -e "$cb/$b" ] && continue           # symlink or real dir already present
+    ln -s "host/$b" "$cb/$b"
+  done
+}
+
 echo "=== [1/3] LIBS (musl bootstrap std) ==="
 run LIBS
 echo "=== [2/3] rustc (musl host) ==="
 run output-1.90.0-$T/rustc
 echo "=== [3/3] cargo (musl host) ==="
-LIBGIT2_SYS_USE_PKG_CONFIG=1 run output-1.90.0-$T/cargo
+# Retry loop: minicargo is incremental, so each retry only redoes the crate that
+# failed onward. `until` keeps the failing build out of `set -e`'s reach.
+attempt=0
+until LIBGIT2_SYS_USE_PKG_CONFIG=1 run output-1.90.0-$T/cargo; do
+  attempt=$((attempt + 1))
+  if [ "$attempt" -gt 4 ]; then
+    echo "!!! cargo build still failing after $attempt attempts" >&2
+    exit 1
+  fi
+  echo "--- cargo build failed; mirroring host OUT_DIRs (attempt $attempt) and retrying ---"
+  symlink_outdirs
+done
 echo "=== DONE ==="
 ls -la output-1.90.0-$T/rustc output-1.90.0-$T/cargo