Slice 8 adds `TraitDecl` / `TraitMethod`. A trait body holds only method
signatures (no default bodies, no associated types or constants); trait
methods reuse `ParamList` / `ReturnType` / `EffectClause` from slice 7,
and the `TraitMethod` node intentionally omits a `Block` child so its
absence is grammar, not a parse error. Diagnostics E0037–E0041.

Slice 9 adds `ImplDecl` after locking todo #4 as the unified `<T>` form
(2026-05-21): every impl is `impl Trait<TypeArgs> { FnDecl* }`. No `on`
keyword, no privileged `self` — single-parameter and multi-parameter
impls share one grammar rule, and the impl body's method signatures
mirror the trait's signatures with type parameters substituted for
concrete types. Impl-body methods reuse `FnDecl` verbatim; the resolver
will distinguish "impl method" from "free function" by walking up to
the enclosing node. Diagnostics E0042–E0046.

Both bodies brace-balance through stray inner blocks (a user writing a
method body inside a trait, or a bare `{...}` inside an impl) so the
surrounding body loop doesn't lose sync with its own closing `}`.

Test layout: ck_parser tests split by production
(`src/tests/{data,fn,trait,impl}_decl.rs` + shared helpers and
cross-cutting tests in `src/tests/mod.rs`) so every file stays under
the R3 line cap as productions land. New worst case: `lib.rs` at 798.

New `.cktest` fixtures under `tests/parser/trait/` (6) and
`tests/parser/impl/` (8) cover happy paths and each new error code.
The CST round-trip set is extended with trait + impl snippets.

@@ -0,0 +90,4 @@
+        }
+        self.expect(SyntaxKind::Eq, "E0054", "expected `=` in let binding");
+        self.parse_expr(0);
+        self.expect(

@@ -0,0 +10,4 @@
+use super::*;
+use ck_lex::SyntaxKind;
+
+fn first_block(out: &ParseOutput) -> &SyntaxNode {