Quadratic run time when checking a start tag for duplicate attribute names

Summary

`BytesStart::attributes()` returns an `Attributes` iterator which, by default (`with_checks(true)`), rejects a start tag that repeats an attribute name. For each attribute yielded, the iterator compared the new name against every name seen so far in the same tag using a linear scan, so a start tag with `N` distinct attribute names cost `O(N²)` byte comparisons. There was no bound on `N` other than the size of the buffered start tag.

## Impact

Any code that parses untrusted XML and iterates a start tag's attributes with the default duplicate check enabled can be made to spend CPU time quadratic in the number of attributes on a single tag. Because the check is pure computation with no `.await`/I/O, an I/O-based timeout on the consumer (for example a read or request timeout) cannot interrupt it while it runs.

Measured cost of a single start tag, release build:

| Attributes on one tag | Time | |---|---| | 80,000 | ~6 s | | 800,000 | ~10 min |

The cost grows with the square of the attribute count, so a start tag of a few tens of megabytes can stall a parsing thread for hours. No memory is exhausted and the parser does not crash; the effect is CPU exhaustion on the thread doing the parsing: a single crafted start tag can pin a CPU core for minutes to hours, denying service to that worker. A deployment that places a wall-clock bound on parsing, or confines it to a non-critical thread, may consider the availability impact lower.

## Affected code paths

* `BytesStart::attributes()` / `Attributes` iterated with checks enabled (the default), and `BytesStart::try_get_attribute`. * `NsReader`, which resolves namespaces by iterating a tag's attributes and so reaches the same check internally.

Consumers that iterate attributes with `.attributes().with_checks(false)` and do not use `NsReader` are not affected.

This was reported as reachable by a remote, unauthenticated attacker in a real-world RPKI relying party (NLnet Labs Routinator) via a crafted RRDP `snapshot.xml`.

## Remediation

Upgrade to `quick-xml >= 0.41.0`, where the duplicate check keeps the linear scan for start tags with a small number of attributes and switches to an `O(1)` hash pre-filter above a threshold, making the whole tag `O(N)`. The reported `AttrError::Duplicated` positions are unchanged.

If upgrading is not possible and duplicate-name detection is not required, disable it with `.attributes().with_checks(false)` (this does not help `NsReader` consumers, which have no equivalent opt-out before 0.41.0).