Fix accessor vtable dispatch through base-typed references

[ASDAlexander77]

Summary

• Closes the "much bigger gap" flagged alongside super-accessor-crash-fix (Fix cross-module super-accessor crash and add class coverage #278): overriding a get/set accessor pair in a subclass and accessing it through a BASE-TYPED reference (an ordinary upcast, not super) always resolved statically to the base's own accessor instead of dispatching virtually to the override - a real correctness gap (standard OOP virtual dispatch), unlike ordinary methods which already dispatch through the vtable correctly.
• Root cause: an accessor's backing get/set functions are ALREADY registered as ordinary MethodInfo entries in ClassInfo::methods (same list ordinary methods use) with a real vtable slot assigned via registerClassMethodMember/getVirtualTable - accessors are virtual by default exactly like methods. But ClassAccessorAccess never checked isVirtual at all: it unconditionally built ThisAccessorOp with a hardcoded FlatSymbolRefAttr naming THIS class's own get/set symbol, completely bypassing the vtable that was already being correctly built.
• Fix: mirror ClassMethodAccess's own isVirtual/isStorageType check (isStorageType is true for super.<accessor>'s raw-struct thisValue, so super access is naturally excluded from virtual dispatch, same as methods) and route through the vtable via VirtualSymbolRefOp (the same op static virtual method access already uses) feeding into the existing "indirect" (function-value) ThisIndirectAccessorOp, instead of the hardcoded-symbol ThisAccessorOp. Checked before the isDynamicImport branch, matching ClassMethodAccess's ordering.
• One regression found and fixed during verification: getThisRefOfClass only narrows thisValue for the isSuperClass case, so an ordinary access whose static type is still a wider union (e.g. T | null, narrowed only at the type-info level, never cast at the value level - hit by 00iterator_bug.ts's spread-operator-generated .length access) reached the new vtable lookup with an un-narrowed union type, which mlirGenPropertyAccessExpression's PropertyRefOp rejects (unlike the old AnyType-typed ThisAccessorOp it replaces). Fixed by explicitly casting to classInfo->classType before the vtable lookup.

Test plan

• New same-file regression test 00class_accessor_virtual.ts (compile + jit tiers)
• Closed the historical NOTE comment in import_class_accessor.ts documenting this exact gap as deliberately untested - now asserts virtual dispatch through a base-typed reference works, cross-module, across all 3 tiers (plain, -shared compile, -jit -shared)
• Full suite: ctest -C Debug -j8 — 810/810 green (808 previously green + 2 new; caught and fixed a 00iterator_bug.ts regression mid-development, see commit message)

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