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refactor(deque): skip the survivor shift on a tail drain, tighten two comments

Follow-up to #4189 from its review.

`Deque::drain` handled a drain ending at the end of the front run with one
`else` arm: shift the front survivors right by the drained length and advance
`head`. That is needed when something survives after the drained range -- the
rest of the front, or a non-empty back run -- so the survivors stay contiguous
with it. When the deque is unwrapped and the drain takes its tail, nothing
survives after the range, the survivors already sit at `[head, head + start)`,
and the shift was O(start) work that moved every survivor one slot for no
reason. The arm is now guarded on `len < front_max_drain || back.length() != 0`
and the tail case falls through with `head` untouched.

A new test pins the behaviour through a view taken before the drain: with the
survivors left in place the view still reads them at their original offsets,
whereas the old shift made it read the first survivor twice. Reverting the
guard fails that test and nothing else.

Two comments in `clear` and `truncate` called "an emptied deque restarts at
offset zero" an invariant. It is not one: `pop_front` and `extract_if` can
empty a deque without resetting `head`, and nothing depends on it. They now say
the three functions arrange it, which is what they do.

Also adds a `release_unused` test where the unused capacity sits on both sides
of an unwrapped run, so each of the two `fill_slots` calls in the unwrapped
branch is pinned by a test on its own.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WjN1QF3HbJohHocMPBRcuw

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