Raise FeatureNotAvailable for unsupported comprehension targets

[gaoflow]

Summary

Fixes #76. A list/dict comprehension whose for clause uses a non-name assignment target — a subscript like x[0] or an attribute like x.attr — crashes with a raw, non-SimpleEval exception:

>>> EvalWithCompoundTypes().eval("[x for x in ([None],) for x[0] in (15,)]")
AttributeError: 'Subscript' object has no attribute 'elts'

recurse_targets only special-cased ast.Name and assumed every other target was a tuple/list with an .elts attribute.

Fix

Handle ast.Name and ast.Tuple/ast.List explicitly, and raise the library's own FeatureNotAvailable for any other target type. This resolves the actual complaint in the issue (a raw AttributeError leaking instead of a SimpleEval exception).

I deliberately did not implement subscript assignment (container[key] = value). The same target path is what enables the self-referential-list construction the reporter flagged in the thread:

>>> [x for x in ([None],) for x[0] in (x,)]   # CPython: [[[...]]]

i.e. a list that contains itself — a potential DoS vector (e.g. RecursionError on comparison). For a sandboxing evaluator, rejecting these targets is the safer behaviour, and it keeps attribute/subscript writes out of the sandbox (attribute writes were never supported). After the fix both the subscript and attribute target forms raise FeatureNotAvailable.

Verification

• Reproduced on main (afd1ffc): subscript target raises a raw AttributeError; after the fix it raises FeatureNotAvailable, as does the self-referential construction and an attribute target.
• All existing name / tuple / list / nested-unpack / dict-comprehension paths are unchanged.
• Added 3 regression tests (test_subscript_target_fails_cleanly, test_attribute_target_fails_cleanly, test_self_referential_target_blocked); each fails before the change (raw AttributeError) and passes after.
• Full test suite: 162 passed (159 + 3). ruff==0.6.9 and mypy==1.11.2 clean.

---

This pull request was prepared with the assistance of AI, under my direction and review.