iter str-concat: heap-Str concatenation primitive in four-site lockstep

Closes fieldtest-form-a friction finding #4. `str_concat : (borrow Str,
borrow Str) -> Str` ships in the four-site-lockstep pattern established
by `str_clone` / `int_to_str` / `bool_to_str` (iter 24.1). The
LLM-natural Show-MyType body
`(app str_concat "label=" (app int_to_str x))` now parses, checks,
builds, and runs end-to-end.

Sites touched (lockstep):
- runtime/str.c — `ailang_str_concat(a, b)` slab-allocates and
  memcpys both source payloads into a new heap-Str.
- ailang-check/src/builtins.rs — `env.globals.insert("str_concat",
  Fn { 2x Str borrow, ret Str own, effects [] })` + `list()` row +
  `install_str_concat_signature` unit test.
- ailang-codegen/src/lib.rs — `declare ptr @ailang_str_concat(ptr,
  ptr)` extern + `lower_app` arm after str_clone + `is_builtin_callable`
  extension + IR-pin unit test `str_concat_emits_call_to_ailang_str_concat`.
- examples/show_user_adt_with_label.ail (new) + crates/ail/tests/
  str_concat_e2e.rs (new) — corpus fixture exercising the LLM-natural
  Show body shape + E2E pin asserting check + build + run produce
  `Item 42\n`.

Lockstep collision repaired: examples/bug_unbound_in_instance_method.ail
used `str_concat` as its UNBOUND name (because that was the literal
fieldtester repro). Renamed to `format_label` (LLM-author-realistic
helper name that will never become a builtin) and updated the pin test
`crates/ail/tests/unbound_in_instance_method_pin.rs` accordingly,
preserving the regression guard's intent (instance-method-body walked
through unbound-var check).

DESIGN.md amended: new §"Heap-Str primitives" subsection between the
milestone-24 Show-backer enumeration and the existing
`Primitive output goes through ...` paragraph, cataloguing all five
heap-Str primitives (`int_to_str`, `bool_to_str`, `float_to_str`,
`str_clone`, `str_concat`) with signatures, iter origins, and the
user-visible-vs-prelude-internal distinction. Show-backer block
unchanged.

IR snapshots regenerated (hello, list, max3, sum, ws_main) to absorb
the new `declare ptr @ailang_str_concat(ptr, ptr)` line in the
unconditional extern header — same upkeep pattern as hs.4 which
regenerated the same 5 snapshots for the same reason
(unconditional declares dead-stripped by clang -O2 when unused).

Tests: 559 + 3 = 562 green (E2E pin + builtin-signature test + IR-pin
test). Zero re-loops across all 7 tasks.

crates/ail/tests/unbound_in_instance_method_pin.rs

@@ -14,11 +14,11 @@
 //! Pre-fix observed behaviour (the bug):
 //! - `ail check` exits 0 with `ok (23 symbols across 2 modules)`.
 //! - `ail build` exits 1 with
-//!   `Error: monomorphise_workspace: unknown identifier: \`str_concat\``.
+//!   `Error: monomorphise_workspace: unknown identifier: \`format_label\``.
 //!
 //! Post-fix expected behaviour:
 //! - `ail check` exits 1 with an `[unbound-var]` error naming
-//!   `str_concat`. The mono pass never runs because check fails first.
+//!   `format_label`. The mono pass never runs because check fails first.
 //!
 //! Root cause (from debugger Phase 1-2):
 //! `crates/ailang-check/src/lib.rs::check_def` early-returns `Ok(())`
@@ -30,9 +30,9 @@
 //! method-body identifier graph.
 //!
 //! Fixture: `examples/bug_unbound_in_instance_method.ail`. The fixture
-//! uses `str_concat` (NOT a builtin) inside an instance-method lambda;
+//! uses `format_label` (NOT a builtin) inside an instance-method lambda;
 //! `int_to_str` IS a builtin and is correctly resolved. The fixture
-//! parses cleanly — the only error is the unbound `str_concat` in the
+//! parses cleanly — the only error is the unbound `format_label` in the
 //! method body.
 
 use std::path::Path;
@@ -43,13 +43,13 @@ fn ail_bin() -> &'static str {
 }
 
 /// RED today: `ail check` on `bug_unbound_in_instance_method.ail`
-/// must exit non-zero and emit `[unbound-var]` naming `str_concat`,
+/// must exit non-zero and emit `[unbound-var]` naming `format_label`,
 /// matching the diagnostic shape produced at fn-body level.
 ///
 /// Pre-fix, this test fails because `ail check` exits 0 and prints
 /// `ok (23 symbols across 2 modules)`.
 #[test]
-fn check_fires_unbound_var_for_str_concat_in_instance_method_body() {
+fn check_fires_unbound_var_for_format_label_in_instance_method_body() {
     let manifest_dir = env!("CARGO_MANIFEST_DIR");
     let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
     let src = workspace
@@ -81,8 +81,8 @@ fn check_fires_unbound_var_for_str_concat_in_instance_method_body() {
          got: {combined}"
     );
     assert!(
-        combined.contains("str_concat"),
-        "diagnostic must name the unbound identifier `str_concat`; \
+        combined.contains("format_label"),
+        "diagnostic must name the unbound identifier `format_label`; \
          got: {combined}"
     );
     assert!(
@@ -128,9 +128,9 @@ fn check_json_unbound_var_in_instance_method_body() {
         arr.iter().any(|d| {
             d.get("severity").and_then(|v| v.as_str()) == Some("error")
                 && d.get("code").and_then(|v| v.as_str()) == Some("unbound-var")
-                && d.to_string().contains("str_concat")
+                && d.to_string().contains("format_label")
         }),
         "expected an error diagnostic with code `unbound-var` naming \
-         `str_concat`; got: {stdout}"
+         `format_label`; got: {stdout}"
     );
 }