iter clippy-sweep: clear all 61 cargo clippy warnings

Hygiene sweep across the workspace under `cargo clippy --workspace
--all-targets`. Before: 61 warnings. After: zero. Tests stay 563
green, `cargo doc` stays at zero warnings, and all three bench
scripts exit 0 against existing baselines.

Twelve lint classes, three fix shapes:

- Documentation hygiene (~32 hits): `doc_lazy_continuation` resolved
  by adding blank lines before continuation paragraphs or rephrasing
  the offending line; plus four `empty_line_after_doc_comments` hits
  in workspace.rs where eight `///` blocks left orphan by form-a.1
  T5 test relocation were converted to plain `//`.
- Idiomatic refactors (~16 hits): `.err().expect()` → `.expect_err()`,
  redundant `.into_iter()` and `.into()` removed, `|f| g(f)` →
  `g`, `.trim().split_whitespace()` → `.split_whitespace()`,
  `push_str("x")` → `push('x')`, two manual `impl Default` flipped
  to `#[derive(Default)]` + `#[default]`, two nested `if let Some(_)
  = …` collapsed.
- Block extraction (1 hit): a 13-line block inside an `else if`
  condition in synth's Var-arm extracted to a new helper
  `is_class_method_dispatch(name, env)` next to
  `qualifier_is_class_shape`.

Three `#[allow]`s with inline rationale where clippy's suggestion
would lose meaning: `only_used_in_recursion` on walk_pattern (the
parameter preserves the five-fn walker-framework signature
uniformity), 2× `if_same_then_else` on qualify_local_types shapes
(two branches encode semantically distinct disqualification
reasons), `too_many_arguments` on `Emitter::new` (8 workspace-flat
tables; bundling would just rename boilerplate).

crates/ailang-core/src/ast.rs

@@ -717,10 +717,11 @@ impl Type {
 ///
 /// `Own` and `Borrow` are author-asserted: the surface form
 /// `(own T)` / `(borrow T)` round-trips through this enum.
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "lowercase")]
 pub enum ParamMode {
     /// Pre-18a / unannotated. Treated as `Own` by the typechecker.
+    #[default]
     Implicit,
     /// `(own T)` — caller transfers ownership; callee consumes.
     Own,
@@ -728,12 +729,6 @@ pub enum ParamMode {
     Borrow,
 }
 
-impl Default for ParamMode {
-    fn default() -> Self {
-        ParamMode::Implicit
-    }
-}
-
 impl ParamMode {
     /// Used by the `skip_serializing_if` predicate on
     /// [`Type::Fn::ret_mode`].