fix: map `\a` escape sequence to BEL character (`\x07`)
The `\a` escape was incorrectly mapped to `\x61` (literal `a`) instead of
`\x07` (BEL), making it inconsistent with the other C-style escapes (`\b`,
`\n`, `\r`, `\t`).
Closes #97
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>