perf(dpi/quic): use Cow<'static, str> for known QUIC version strings

[obchain]

Closes #400

What

QuicInfo::new(version) is called on every QUIC long-header packet. The internal quic_version_to_string helper returned Option<String>, allocating a heap String for each of the seven well-known version identifiers (v1, v2, draft-27/28/29, Q050, T050) — even though they are compile-time constants.

This PR changes QuicInfo.version_string from Option<String> to Option<Cow<'static, str>> and returns Cow::Borrowed for the static arms:

// before
fn quic_version_to_string(version: u32) -> Option<String> {
    match version {
        0x00000001 => Some("v1".to_string()),       // heap alloc
        0x6b3343cf => Some("v2".to_string()),       // heap alloc
        …
    }
}

// after
fn quic_version_to_string(version: u32) -> Option<Cow<'static, str>> {
    match version {
        0x00000001 => Some(Cow::Borrowed("v1")),    // zero alloc
        0x6b3343cf => Some(Cow::Borrowed("v2")),    // zero alloc
        …
        v if (v >> 8) == 0xff0000 => Some(Cow::Owned(format!("draft-{}", v & 0xff))),
        _ => None,
    }
}

QuicInfo derives only Debug + Clone, so the field-type change requires no serde or macro adjustments. Cow::clone() for a Borrowed variant is a pointer copy, so the merge-path clone also becomes cheaper.

The single display callsite in details.rs changes from .as_ref() + .clone() to .as_deref() + .to_owned(), which is semantically identical.

Test

A new unit test quic_version_string_known_versions_are_borrowed asserts:

• All seven known versions produce Cow::Borrowed — enforcing the no-alloc guarantee at the type level.
• An unknown version produces None.
• The dynamic draft-N arm produces Cow::Owned with the correct label.

Impact

Per QUIC long-header packet with a known version: −1 heap allocation. Under sustained HTTP/3 traffic (QUIC v1) this path is hot — every packet avoids a small-string allocation and its associated allocator overhead.

Verification

cargo build --workspace   # clean
cargo test -p rustnet-core network::dpi::quic  # 12 passed
cargo test -p rustnet-core quic_version_string  # 1 passed
cargo fmt --check         # clean
cargo clippy --all-targets -- -D warnings  # clean