fix(parser): fall back from (( … )) arith to nested subshells

[mpecan]

Summary

• Adds the ((…)) → nested-subshell fallback that bash's parser performs when the body of ((…)) is not a valid arithmetic expression. Input like ((x\n> 0)\n) is now parsed as two nested subshells instead of failing with expected )), got Eof.
• Introduces a small LexerCheckpoint save/restore API (pos, line, peeked, last_token_end) on Lexer. The arith dispatch in parse_command_inner now tries parse_arith_command first and, on a RableError::MatchedPair from read_until_double_paren, restores the checkpoint and re-parses via parse_subshell. Other arith-side errors still propagate.
• Removes reserved_word_as_word 2 from KNOWN_ORACLE_FAILURES in tests/integration.rs.

Closes #42.

Test plan

• cargo fmt
• cargo clippy --all-targets -- -D warnings — no warnings
• cargo test — 238 passed, 0 failed
• New unit tests in src/parser/tests.rs:
  • arith_command_with_inner_parens — (( (1+2) )) still parses as arithmetic
  • double_paren_falls_back_to_nested_subshells — the exact issue input
  • double_paren_fallback_preserves_following_command — lexer state is clean after fallback
  • double_paren_fallback_inside_if — fallback works when dispatched from inside a compound construct
• reserved_word_as_word 2 now passes in oracle_bash_valid_divergences