Arithmetic: recursive variable deref, array access, quoted substitution not implemented

[chaliy]

Description

Several dynamic arithmetic features are missing.

Scope

• Recursive variable dereferencing: b=a; a=3; echo $((b+1)) should output 4 (deref b to a to 3), but outputs 1
• Variable expression re-evaluation: x='1 + 2'; echo $((x * 3)) should output 9 (re-evaluate x as expression), but outputs 7
• Array access in arithmetic: arr=(10 20 30); echo $((arr[1])) should output 20, but outputs 0
• Array index expressions: echo $((arr[1+1])) should work
• Quoted substitution: echo $(("$x" * 3)) should work

Repro

a=3
b=a
echo $(( b + 1 ))
# bashkit outputs: 1 (wrong — treats 'a' as 0)
# expected: 4 (dereferences b->a->3)

arr=(10 20 30 40)
i=2
echo $(( arr[i] ))
# bashkit outputs: 0 (wrong)
# expected: 30

Test coverage

5 skipped tests in crates/bashkit/tests/spec_cases/bash/arith-dynamic.test.sh (PR #351).

Oils reference: https://github.com/oilshell/oil/blob/master/spec/arith-dynamic.test.sh