Iter 6: deps hardening, multi-diagnose, DESIGN audit

- ail deps now filters builtins, fn params, and let/match-pattern
  bindings. New value_names() in ailang-check::builtins is the single
  source of truth shared with the typechecker install path. walk_term
  threads a scope set; qualified `prefix.def` refs pass through.
- check_in_workspace returns Vec<CheckError>; check_workspace
  accumulates body diagnostics across defs and modules. Pass-1
  (top-level symbol table) and per-module type-def setup stay
  fail-fast — corrupt env would taint later diagnostics.
- DESIGN.md "What the MVP is NOT" was lying (ADTs, strings landed
  in Iter 2/3). Renamed to "What is not (yet) supported" and split
  into "not yet" + supported/smoke-tested. JOURNAL Iter 6 entry
  records the architecture self-check.

Tests: 47 green (was 44). +2 deps filter tests in e2e,
+1 multi-diagnose test in ailang-check workspace integration.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/ail/tests/e2e.rs

@@ -349,6 +349,73 @@ fn describe_workspace_resolves_qualified_name() {
     );
 }
 
+/// Guards Iter 6: `ail deps` no longer surfaces built-ins, params, or
+/// let-bindings as edges. The single-module `sum` def in `sum.ail.json`
+/// uses `+`, `-`, `==`, the param `n`, and the recursive call `sum`.
+/// Only the recursive call must remain.
+#[test]
+fn deps_filters_builtins_params_locals() {
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
+    let src = workspace.join("examples").join("sum.ail.json");
+
+    let output = Command::new(ail_bin())
+        .args(["deps", src.to_str().unwrap(), "--of", "sum", "--json"])
+        .output()
+        .expect("ail deps failed to run");
+    assert!(
+        output.status.success(),
+        "ail deps exited non-zero; stderr: {}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+
+    let stdout = String::from_utf8(output.stdout).expect("stdout utf8");
+    let v: serde_json::Value = serde_json::from_str(stdout.trim()).expect("valid JSON");
+    let arr = v.as_array().expect("array of {name, refs}");
+    let entry = arr
+        .iter()
+        .find(|e| e["name"].as_str() == Some("sum"))
+        .expect("sum entry present");
+    let refs: Vec<&str> = entry["refs"]
+        .as_array()
+        .expect("refs array")
+        .iter()
+        .filter_map(|x| x.as_str())
+        .collect();
+    assert_eq!(
+        refs,
+        vec!["sum"],
+        "expected only the recursive call `sum`; got {refs:?}"
+    );
+}
+
+/// Guards Iter 6 in workspace mode: edges into built-ins (`ws_lib.+`)
+/// and into the params (`ws_lib.a`, `ws_lib.b`) of `ws_lib.add` must be
+/// gone after the deps refactor.
+#[test]
+fn deps_workspace_filters_builtins_and_params() {
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
+    let entry = workspace.join("examples").join("ws_main.ail.json");
+
+    let output = Command::new(ail_bin())
+        .args(["deps", entry.to_str().unwrap(), "--workspace", "--json"])
+        .output()
+        .expect("ail deps --workspace failed to run");
+    assert!(output.status.success());
+
+    let stdout = String::from_utf8(output.stdout).expect("stdout utf8");
+    let v: serde_json::Value = serde_json::from_str(stdout.trim()).expect("valid JSON");
+    let edges = v["edges"].as_array().expect("edges array");
+
+    for e in edges {
+        let to_def = e.get("to_def").and_then(|x| x.as_str()).unwrap_or("");
+        assert_ne!(to_def, "+", "built-in `+` must not appear: {e}");
+        assert_ne!(to_def, "a", "param `a` must not appear: {e}");
+        assert_ne!(to_def, "b", "param `b` must not appear: {e}");
+    }
+}
+
 /// Guards Iter 5d: `ail deps --workspace` contains a cross-module edge
 /// `ws_main.main -> ws_lib.add`, because `ws_main` calls `ws_lib.add(2,3)`.
 #[test]