test(series): RED pin for polymorphic RawBuf element-var forwarding

Base-tier enabling RED for the `series` milestone step 1 (#61). A
polymorphic fn that allocates `(new RawBuf n)` with no explicit
type-arg — element var solved purely from the enclosing ctor-field
type `(con RawBuf a)`, never from a value argument — checks clean but
fails `ail build --alloc=rc`: mono drops the check-phase solution for
`a`, defaults it to Unit, and codegen mints the unregistered
`RawBuf_new__Unit`. The fixture reads the element back via `RawBuf.get`
so a wrong specialisation is observable (the program must print the
stored `1.0`).

This is the precondition the user picked (option B) so Series can ship
as a pure AILang library extension over RawBuf while keeping the
model-0007 `(new Series (con Float) 3)` caller surface. The #51 fix
carried a WRITTEN `(con Int)` type-arg through mono; the
variable-via-expected-type case is the untouched gap.

RED: `ail check` exits 0; `ail build --alloc=rc` aborts with
`RawBuf_new__Unit has no registered intercept`. GREEN target: builds,
runs, prints `1.0`. The mono fix must not break the INTERCEPTS↔
(intrinsic) bijection or the Term::New↔pre_desugar_validation lockstep.

refs #61

crates/ail/tests/raw_buf_new_poly_elem_pin.rs

@@ -0,0 +1,83 @@
+//! RED-pin for the series-step-1 enabling fix (#61, base-tier): a
+//! polymorphic function may allocate a `RawBuf a` over its OWN forall
+//! type variable via `new` with NO explicit type-arg — the element
+//! type is solved purely from the expected ctor-field context
+//! `(con RawBuf a)` — and under monomorphisation the buffer must
+//! specialise to the CALLER's concrete element type, NOT default to
+//! Unit.
+//!
+//! Property protected: a check-time-solved element var on
+//! `(new RawBuf n)` survives monomorphisation. When the only carrier
+//! of the buffer's element type is the enclosing ADT ctor-field type
+//! `(con RawBuf a)` (the builder `mk` takes only the size, never a
+//! value of type `a`), mono must propagate the check-phase solution
+//! for `a` to the caller's concrete instantiation. Today it drops
+//! that solution, defaults `a` to Unit, and codegen mints
+//! `RawBuf_new__Unit`, which has no registered intercept
+//! (crates/ailang-codegen/src/intercepts.rs registers only
+//! Int/Float/Bool). The fix lives in mono substitution propagation to
+//! Term::New lowering (crates/ailang-check/src/lower_to_mir.rs:501,
+//! `elem: None`). Precedent: the #51 explicit-`(con Int)`-type-arg
+//! fix carried a WRITTEN annotation through mono; the
+//! variable-via-expected-type case here is the untouched gap.
+//!
+//! This is a CLI-boundary E2E test because the defect spans the
+//! check -> mono -> codegen pipeline and the symptom is the exit
+//! behaviour of `ail build` itself. The element is read back via
+//! `first` (RawBuf.get) so the specialisation is OBSERVABLE: a wrong
+//! specialisation could not print the stored Float.
+
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+fn repo_root() -> PathBuf {
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    Path::new(manifest_dir)
+        .parent()
+        .unwrap()
+        .parent()
+        .unwrap()
+        .to_path_buf()
+}
+
+/// A polymorphic `Box a` whose builder `mk` allocates `(new RawBuf n)`
+/// over its own forall var `a` (no type-arg, element solved from the
+/// `(con RawBuf a)` ctor-field context). Instantiated by the caller at
+/// `Float`, it must build under `--alloc=rc` and print the stored
+/// `1.0` — proving the inner buffer specialised to the caller's
+/// element type rather than defaulting to Unit.
+#[test]
+fn rawbuf_new_poly_elem_specialises_to_caller_type() {
+    let fixture = repo_root().join("examples/raw_buf_new_poly_elem.ail");
+    let tmp = std::env::temp_dir().join(format!(
+        "ailang_rawbuf_new_poly_elem_{}",
+        std::process::id()
+    ));
+    std::fs::create_dir_all(&tmp).unwrap();
+    let out = tmp.join("bin");
+
+    let build = Command::new(env!("CARGO_BIN_EXE_ail"))
+        .args(["build", fixture.to_str().unwrap(), "--alloc=rc", "-o"])
+        .arg(&out)
+        .output()
+        .expect("ail build failed to spawn");
+    assert!(
+        build.status.success(),
+        "ail build --alloc=rc failed for polymorphic RawBuf<a> over a \
+         forall var solved from the ctor-field context: stderr={}",
+        String::from_utf8_lossy(&build.stderr)
+    );
+
+    let run = Command::new(&out).output().expect("execute binary");
+    assert!(
+        run.status.success(),
+        "binary exited non-zero: {:?}",
+        run.status
+    );
+    let stdout = String::from_utf8_lossy(&run.stdout);
+    assert!(
+        stdout.trim() == "1.0",
+        "expected the program to print the stored Float `1.0` (buffer \
+         specialised to the caller's element type Float), got: {stdout:?}"
+    );
+}