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Add reversed() support to MultiDict views (C-ext + pure-Python) (#1340)

## What
Adds `__reversed__` to the keys, values, and items views of `MultiDict`,
`CIMultiDict`, and their proxies in **both** the C-extension and the
pure-Python implementation.

## Why
`dict` views have been reversible since Python 3.8; `multidict` views
supported forward iteration only. This brings parity for common use
cases (walking headers in reverse insertion order, popping the most
recently inserted occurrence) without forcing users to materialize a
list first.

Implements #448.

## How
- **C-extension** (`_multilib/hashtable.h`): added `md_init_pos_reverse`
and `md_prev`, mirroring `md_init_pos` / `md_next` (skipping `NULL`
identity slots left by deletions).
- **C-extension** (`_multilib/iter.h`): the iterator struct gained a
`reverse` flag; the existing per-kind `*_iter_iternext` functions
dispatch on it. Reverse iterators reuse the existing iterator
types/specs, so the existing `multidict_type_leak*` tests continue to
cover them.
- **C-extension** (`_multilib/views.h`): each view exposes
`__reversed__` (`METH_NOARGS`) returning a reverse-iterator instance.
- **Pure-Python** (`_multidict_py.py`): `HtKeys.iter_entries_reverse`
filters the entry array in reverse; each view class gained
`__reversed__` and a paired `_iter_reversed` that honors the version
guard.

## Testing
- New tests in `tests/test_multidict.py` cover read-only cases
(keys/values/items/empty/`__length_hint__`) parametrized over
`MultiDict`, `CIMultiDict`, both proxy variants, and both
implementations.
- New tests in `tests/test_mutable_multidict.py` cover skipping `NULL`
slots after `del`, and the `RuntimeError` raised when the dict is
mutated during iteration.
- Full suite: `1706 passed in 7.16s` (Python 3.12, C-ext + pure-Python).
- `pre-commit` (ruff, clang-format, mypy on 3.11 + 3.13): clean.

## Notes
This PR supersedes #1304 (which added pure-Python support only and was
waiting on a C implementation ... (continued)

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