Improve typing coverage with ParamSpec + Protocol

[toloco]

Summary

• Added CachedCallable[P, R] protocol — preserves the original function's call signature while exposing cache_info() and cache_clear()
• Added BaseCacheInfo protocol — common interface for cache info objects (hits, misses, max_size, current_size)
• cache() return type updated from Callable[[F], F] to Callable[[Callable[P, R]], CachedCallable[P, R]]
• Added _probe to _warp_cache_rs.pyi stub (removes type: ignore comments)
• typing_extensions dependency for Python 3.9 only

Closes #10

Before

@cache(max_size=128)
def add(x: int, y: int) -> int: ...

add.cache_info()  # type error: no attribute 'cache_info'

After

@cache(max_size=128)
def add(x: int, y: int) -> int: ...

add(1, 2)          # type: int
add.cache_info()   # type: BaseCacheInfo
add.cache_clear()  # OK

Test plan

• ty check passes with zero diagnostics
• ruff check passes
• All 79 existing tests pass
• New exports verified: from warp_cache import CachedCallable, BaseCacheInfo

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