Improve PostgreSQL acceptance and sync README benchmark snapshot

[yigalrozenberg]

Summary

• improve PostgreSQL parse tolerance for ON CONFLICT expression targets and additional operator sequences
• add parser regression tests for these PostgreSQL cases
• rerun full all-dialect benchmark and sync README acceptance snapshot and architecture/version docs

Validation

• cargo test (targeted parser/integration/transpile tests for affected behavior)
• full benchmark: sqlglot_rust_bench all 6
  • total acceptance: 97.65% (304270/311594), panics: 0