B031: don't count store-context references as generator uses

[StressTestor]

what

fixes a B031 false positive: type-annotating (or assigning to) the groupby loop variable was counted as a "use", so a single real use plus an annotation tripped "used more than once".

for _, group in groupby(items):
    group: SomeType        # annotation, not a use
    do_something(group)    # the only real use

before: B031 fires. after: clean.

why

the B031 usage-counter walked the loop body and counted every Name matching the group variable regardless of context. an annotation target (group: T) or an assignment target is a Store-context Name, not a use of the generator, so it inflated the count. count only Load-context references.

scope

this is the type-annotation / store-context case raised in the comments on #465. the original if/else case is handled separately by #557, and the two are orthogonal: if/else uses are both loads, untouched by this change, so they don't collide. real double-uses still fire B031.

tests

a case in tests/eval_files/b031.py that annotates the loop variable then uses it once, expecting no B031. it fails before the fix and passes after. full suite stays green.

Addresses #465.