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locker: add Done/Do/Prefixed helpers and make Factory an interface

Done() returns a channel closed when the Locker is no longer holding
the lock — either after Unlock or when the backend session is lost
(TTL elapsed without renewal, connection dropped). The channel
reflects the most recent successful Lock/TryLock; calls before any
acquire return an already-closed channel. Each driver wires Done to
its existing liveness signal: dummy uses a per-Lock channel closed on
Unlock, tcs reuses the keepalive-failure channel guarded by sync.Once
so Unlock and the keepalive loop never double-close, and etcd returns
concurrency.Session.Done().

Do is the acquire-work-release helper. It takes a Factory, name, work
function, and Options; builds the Locker, acquires it, runs fn while
held, then unlocks — even if fn errors. fn's result is the leading
error so errors.Is against domain sentinels keeps working; Unlock
errors are joined via errors.Join only when fn also failed. README's
distributed-locks section now leads with locker.Do as the recommended
pattern; the manual Lock/Unlock form is reserved for locks that must
outlive a single function call.

Prefixed wraps a locker.Factory to scope every NewLocker name under a
common prefix — mirrors storage.Prefixed but operates on Factory
only, so callers can hand a name-scoped lock binder to a subcomponent
without exposing the full Storage. The prefix is concatenated to the
caller-supplied name with no separator, matching
storage.Prefixed.NewLocker. An empty prefix returns the inner Factory
unchanged. A non-empty prefix must start with "/"
(ErrPrefixNoLeadingSlash) and must not end with "/"
(ErrPrefixTrailingSlash); interior "/" separators are allowed.

Convert Factory from a function type to an interface with a NewLocker
method. Any type with a matching NewLocker satisfies it directly, so
Storage and the Prefixed wrapper need no adapter; FactoryFunc adapts
a bare function for ad-hoc cases (e.g. tests), mirroring
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