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-codegen/src/lib.rs

@@ -154,19 +154,14 @@ type Result<T> = std::result::Result<T, CodegenError>;
 /// Iter 18c once uniqueness inference is wired up.
 /// The IR is otherwise byte-identical between the three strategies
 /// modulo the allocator symbol name.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
 pub enum AllocStrategy {
+    #[default]
     Gc,
     Bump,
     Rc,
 }
 
-impl Default for AllocStrategy {
-    fn default() -> Self {
-        AllocStrategy::Gc
-    }
-}
-
 impl AllocStrategy {
     /// LLVM IR-level name of the allocator fn (without leading `@`).
     fn fn_name(self) -> &'static str {
@@ -759,6 +754,11 @@ struct FnSig {
 }
 
 impl<'a> Emitter<'a> {
+    // The eight parameters are all the workspace-flat tables the
+    // emitter needs at construction time; bundling them into a
+    // context struct would just rename the boilerplate without
+    // reducing it. Keep the explicit-arg shape.
+    #[allow(clippy::too_many_arguments)]
     fn new(
         module: &'a Module,
         module_name: &'a str,