fix: tighten stack validation semantics

[berisher]

Summary

This PR tightens the generator's stack simulation and opcode validation so the
modeled pickle VM stays aligned with real pickle semantics.

• preserve aliasing and object identity across memo operations and BUILD
• reject APPEND, tuple shortcuts, and READONLY_BUFFER in invalid stack states
• fix protocol 0/1 cleanup, proto-0/1 INT bool decoding, and LONG1/LONG4 sign extension
• break isolated StackObjectRef cycles on drop and remove dead StackObject Hash/Eq impls
• add focused unit coverage for the stack/validation edge cases this PR changes

Remediated Findings

• F-0005
• F-0040
• F-0042
• F-0043
• F-0044
• F-0045
• F-0083
• F-0084
• F-0090
• F-0091
• F-0096

How to validate

cargo fmt --check
cargo clippy --all-targets -- -D warnings
cargo test