Fix overriding an inherited field alias back to the field's own name - #1133

Fixes #654.

structmeta_process_rename skips writing to renamed_fields when the requested encode name equals the field's own name, leaving any alias inherited from a base class in place. So field(name="x") on a redeclared field x was silently ignored, and the same held for a rename= policy whose result is the field's own name (e.g. rename="lower" under a parent's rename="upper"):

class Base(Struct):
    x: int = field(name="_x")

class Sub(Base):
    x: int = field(name="x")

Sub.__struct_encode_fields__  # ('_x',), expected ('x',)

The fix clears the inherited entry at both sites instead of skipping the write. Redeclared fields already re-apply the effective naming config, including overriding a base's field(name=...) alias with a policy result; this extends that to identity results.

Two behaviors deliberately unchanged: a redeclaration expressing no naming intent (x: int = 1, no policy in effect) keeps the inherited alias, and so does rename=None, which acts as "no policy in this class" rather than an identity policy. Clearing per field is spelled field(name="x"). Both new tests fail before the fix.