fix: MethodDispatcher.__get__ returns independent bound dispatcher per instance

[gaoflow]

Bug

MethodDispatcher.__get__ stored instance and owner directly on self
(the shared descriptor object) and then returned self:

def __get__(self, instance, owner):
    self.obj = instance   # mutates the descriptor!
    self.cls = owner
    return self            # same object every time

Because every attribute lookup returns the same MethodDispatcher object,
accessing other_obj.method after storing bound = obj.method silently
overwrites self.obj, making bound(...) dispatch to the wrong instance.

Minimal reproducer:

from multipledispatch import dispatch

class Counter:
    def __init__(self):
        self.count = 0

    @dispatch(int)
    def increment(self, x):
        self.count += x

a = Counter()
b = Counter()

bound_a = a.increment
bound_b = b.increment   # overwrites bound_a.obj → a is now b!

bound_a(10)
print(a.count)  # prints 0  ← wrong, should be 10
print(b.count)  # prints 10 ← wrong, should be 0

Fix

__get__ now returns a new BoundMethodDispatcher instance that captures
its own (obj, cls) pair for each lookup, exactly as Python's ordinary
bound-method protocol works. MethodDispatcher itself is no longer
responsible for storing per-call state.

A regression test (test_method_dispatcher_bound_independence) is included.

All 63 tests pass (python -m pytest multipledispatch/tests/ -p no:randomly).

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