JIT: support field access for codon jitclass(4/4)

[LeeLee26]

Description: This PR implements the Field access part mentioned in RFC: Native-backed Python Classes for Codon JIT via codon.jitclass.

Details: Field access is implemented through generated getter and setter methods instead of direct memory access. During class rewriting, each annotated field gets a pair of native methods:

_codon_jitclass_get_<field>()
_codon_jitclass_set_<field>(value)

Python attribute reads and writes are handled by JITField descriptors and lowered into calls to these generated methods. This lets field access reuse the same method-dispatch pipeline, wrapper cache, Python/Codon conversion logic, and safety checks as ordinary method calls.

PR Stack:

1. JIT: add JITCallable to wrap JIT callable functions(1/4) #813
2. JIT: add jitclass for native codon jit class(2/4) #816
3. JIT: add stale jit context detection(3/4) #814
4. JIT: support field access for codon jitclass(4/4) #817 ⬅