Disallow implicit composed-base upcasts; require explicit cast

[marcauberer]

What changed

• Disallow the implicit struct-to-composed-base upcast (a struct composing another struct as its first field is no longer implicitly convertible to that base in assignments/arguments/returns). Interface upcasts are unaffected.
• Allow the explicit cast<Base*>(derived) / cast<Interface*>(struct) in a safe (non-unsafe) context, but only for the upcast direction. The IR generator advances the pointer to the embedded subobject via GEPs, following the first-field composition chain transitively and correctly skipping any vtable prefix added by an implemented interface. Down/sibling pointer casts still require unsafe.
• Migrate the bootstrap compiler (parser, block allocator, generic type) to the explicit cast / explicit composed-field access.

Why

The implicit composed-base upcast was silent and layout-dependent. When a struct also implements an interface, a vtable pointer is placed in front of the composed base, so the base no longer sits at offset 0 — the implicitly emitted identity pointer copy then aliased the vtable region (garbage reads / segfault). Making the conversion explicit keeps pointer identity visible, and routing it through cast<> lets the compiler emit the correct, vtable-aware pointer adjustment.

How it was validated

• spicetest --gtest_filter='TypeCheckerTests.*:IRGeneratorTests.*:BootstrapCompilerTests.*' — all pass except the pre-existing BootstrapCompilerTests.standaloneCommonUtilTest (local version/build-string golden mismatch, unrelated).
• New success test irgenerator/structs/success-explicit-composed-base-cast (cout + IR snapshot) verifies the GEP adjustments: single-level cast skips the vtable (field index 1), two-level cast emits two GEPs (Leaf -> Derived -> Base), and an interface cast stays at offset 0.
• New error test typechecker/structs/error-no-implicit-composed-base-upcast asserts the implicit upcast is rejected.
• Manually confirmed a downcast still requires unsafe.

Follow-up / known limitations

• Down/sibling struct-pointer casts remain unsafe by design (no runtime check).
• The commonUtil bootstrap test failure is pre-existing and environmental.

🤖 Generated with Claude Code

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