Unbounded namespace-declaration allocation in `NsReader` enables memory-exhaustion denial of service

Summary

`NsReader` resolves namespaces by calling `NamespaceResolver::push` for every `Start`/`Empty` event *before* the event is returned to the caller. `push` iterated all `xmlns` / `xmlns:*` attributes on the start tag and, for each one, appended the prefix bytes to an internal buffer and pushed a `NamespaceBinding` (32 bytes on 64-bit) to an internal `Vec`, with no upper bound on the number of declarations.

## Impact

A start tag with `N` namespace declarations drove roughly `3×` the tag's byte size in `NamespaceResolver` heap, allocated *inside* `quick-xml` before the `NsReader` consumer ever received the event and could inspect or reject it. A consumer that bounds its *input* size therefore still cannot bound this allocation: an `M`-byte start tag yields on the order of `3 × M` bytes of resolver heap the caller never sees.

On untrusted XML this lets a remote, unauthenticated attacker force large heap allocations with a single start tag. With several `NsReader`s running concurrently on independent inputs (a common server pattern), the allocations stack and can exhaust process memory, causing the operating system to kill the process (OOM). This was confirmed against a real-world RPKI relying party (NLnet Labs Routinator), where concurrent RRDP validation workers parsing a crafted `snapshot.xml` exceeded the memory limit and the process was OOM-killed.

## Affected code paths

Consumers using `NsReader` (which always calls `NamespaceResolver::push` before yielding `Start`/`Empty`), or calling `NamespaceResolver::push` directly. A plain `Reader` that does not perform namespace resolution is not affected.

## Remediation

Upgrade to `quick-xml >= 0.41.0`. `NamespaceResolver::push` now rejects a start tag that declares more than `DEFAULT_MAX_DECLARATIONS_PER_ELEMENT` (256) namespace bindings, returning the new `NamespaceError::TooManyDeclarations` instead of allocating without limit. The limit is configurable via `NamespaceResolver::set_max_declarations_per_element` (use `usize::MAX` to restore the previous unbounded behavior), and `NsReader::resolver_mut()` is provided to reach it.

There is no clean workaround for `NsReader` consumers before 0.41.0, as the allocation happens inside the reader with no configuration knob to cap it.