feat: report structured compiler error types

[charles-cooper]

co-authored by gpt 5.6 sol

Summary

• add source_error_type and target_error_type to JSON report schema v6
• capture the concrete Vyper exception class at the managed compiler boundary, leaving the fields null when no typed exception is available
• preserve the existing verbatim error text and existing compiler return codes under each attestation's exit_status.code

Fixes #40

[banteg]

Reviewed the schema v6 propagation and managed-compiler error classification. Full local suite (887 tests), focused integration coverage, real Vyper 0.3.10/0.4.3 failure probes, and wheel smoke all pass.