Trap claims survive writes storage cannot serve; extending a rejected base reports at compile time

- trapDeclRootOf's written-binding decline now requires a mappable declared type: ordinary storage only buys anything when it compiles — an unmappable recursive call shape traded the compiling no-storage trap for a guaranteed SC2009 on a value that never exists
- bindingEverWritten skips statement-position writes whose RHS chain roots at an ambient-undefined name: that statement IS the RHS root's throw (lowerExprStatement's first claim) and never touches the target's storage, so it cannot justify the decline
- A poisoned base a top-level class extends flushes its deferred diagnostics eagerly (collectClassShape): the heritage evaluates at module init and the base's fence is the compiler's own — Node defines the base fine — so the deferred trap compiled a binary that refused at startup where Node runs; leaf and derived-only poisoned classes keep the JS deferral
- Fixtures: corpus 2632 (unmappable written trap binding) and 2633 (ambient-rooted rewrite), diagnostics js-extends-poisoned-base (the eager report names the base's real blocker), errors.test pins for both deferral edges that must NOT report eagerly; order-parity baselines recorded for the new entries