refactor: clear clippy code lints across the workspace

Machine-applicable clippy fixes plus three by-hand cleanups; all
semantics-preserving (full workspace test suite green).

Auto-applied (cargo clippy --fix):
- dropped `.clone()` on the Copy type ParamMode (ret_mode/param_mode)
- map_or(false, p) -> is_some_and(p)
- removed redundant closures and an explicit .into_iter()
- useless format!, a needless deref, an as_bytes-after-slice

By hand:
- ailang-codegen: named the loop-frame tuple types (LoopFrame /
  LoopBinderSlot) instead of the bare nested
  Vec<(String, Vec<(String, String, String, Type)>)>, clearing the
  type-complexity lint and documenting what each slot holds.
- ailang-check: collapsed the two identical pass-through arms of the
  Type-Con qualifier (already-dotted / primitive) into one `||` guard.
- ailang-core: converted a stale `///` block (it described a test that
  was relocated, documenting nothing) back to a plain `//` comment.

Remaining clippy output is 16 markdown list-indentation nits in prose
doc comments — doc formatting, not code, left for a docs pass.

crates/ailang-codegen/src/drop.rs

@@ -1005,7 +1005,7 @@ impl<'a> Emitter<'a> {
                 (vars, Type::Con { args, .. })
                     if !vars.is_empty() && vars.len() == args.len() =>
                 {
-                    vars.iter().cloned().zip(args.into_iter()).collect()
+                    vars.iter().cloned().zip(args).collect()
                 }
                 _ => BTreeMap::new(),
             };