iter mq.3: retire MethodNameCollision + multi-class E2E + DESIGN.md sync

Closes the module-qualified-class-names milestone. Deletes the
workspace-load workaround; the two-libraries case now resolves via
type-driven dispatch at the call site with explicit qualifier as the
LLM-author's disambiguation tool. Resolves both mq.2 known-debt items
(active_declared_constraints plumbing, (class,method) re-key). New
synth warnings channel emits class-method-shadowed-by-fn at all three
fn-precedence branches per spec section Class-fn collisions. Three new
E2E fixtures + integration tests cover the ambiguous, qualified, and
class-fn-shadow trajectories. DESIGN.md class-names paragraph rewritten,
new section Method dispatch added. Roadmap P2 marked done, milestone-24
unblocked for re-brainstorm.

9/9 tasks. 545 tests green. bench/compile_check.py + cross_lang.py
exit 0; bench/check.py exit 1 (2 noise-class regressions, runtime
uncoupled to typecheck iter).

crates/ail/src/main.rs

@@ -1198,25 +1198,6 @@ fn workspace_error_to_diagnostic(
                 "var": var,
             })),
         ),
-        W::MethodNameCollision {
-            method,
-            kind,
-            first_origin,
-            second_origin,
-        } => Some(
-            ailang_check::Diagnostic::error(
-                "method-name-collision",
-                format!(
-                    "method name `{method}` collides ({kind}): defined in `{first_origin}` and `{second_origin}`"
-                ),
-            )
-            .with_ctx(serde_json::json!({
-                "method": method,
-                "kind": kind,
-                "first_origin": first_origin,
-                "second_origin": second_origin,
-            })),
-        ),
         W::MissingSuperclassInstance {
             class,
             superclass,