feature/constant propagation v2

[nahime0]

Summary

Implements constant-propagation-v2 with more precise local propagation and folding:

• use known switch subjects to merge only reachable switch paths
• ignore unreachable catch writes when the propagated try body is non-throwing
• fold known match expressions before assignment propagation
• fold scalar indexed and associative array-literal accesses
• update README, optimizer docs, pipeline docs, and ROADMAP

Tests

• cargo test constant_propagation
• cargo test -- --include-ignored