Implement ? operator for error propagation

[ochsec]

Summary

• Add postfix ? operator that unwraps Result/Option values, returning early on Err/None — mirrors Rust's ? operator
• Handles interaction with pipe operator (x |> F? |> G?) by unwrapping Propagate during pipe desugaring, then re-wrapping
• Binds tighter than binary ops and pipes (1 + x? parses as 1 + (x?))

Changes

• Lexer: Question token for ?
• AST: Propagate { expr } expression variant
• Parser: Postfix ? handling after primary expressions, function calls, and binary op operands; pipe desugaring updated to handle Propagate-wrapped RHS
• Codegen: Propagate { expr } transpiles to (expr)? in Rust
• Type inference: Verifies ? is applied to Option<T> or Result<T, E>, returns unwrapped T
• 14 new tests covering lexer, parser, and codegen
• Example file compiler/examples/error_propagation.w

Test plan

• cargo build passes
• cargo test — all 198 tests pass (14 new + 184 existing)
• Verified ? works with: bare identifiers, function calls, pipe chains, binary ops, Some/Ok expressions

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