feat(interpreter): implement recursive variable deref and array access in arithmetic

[chaliy]

Summary

• Implement recursive variable dereferencing in arithmetic context: bare variable names are recursively resolved until a numeric value is found (e.g. b=a; a=3; $((b+1)) yields 4)
• Add expression re-evaluation: variable values containing arithmetic expressions are evaluated as sub-expressions with proper grouping (e.g. x='1+2'; $((x*3)) yields 9)
• Add array element access in arithmetic: arr[expr] evaluates the index expression and looks up the array element
• Handle double-quoted substitution in arithmetic context by stripping quotes

Closes #361

Test plan

• Enabled 5 previously-skipped spec tests in arith-dynamic.test.sh
• All 1015 library unit tests pass
• All bash spec tests pass (1184 passed, 0 failed)
• Bash comparison tests pass (vs real bash)
• cargo fmt --check clean
• Pre-existing clippy warning unrelated to this change (resolve_redirect_url dead code)