Add rust-hpke X-Wing benchmarks (via v0.14.0-pre.2) and update README coverage

[danieldia-dev]

This PR makes two improvements to the comparative benchmark suite:

1. rust-hpke is added to all previously-omitted X-Wing benchmark groups by bumping the dev-dependency from hpke v0.13 to a v0.14 pre-release (git pin, rev = 5c682cf2). With v0.14, the base gen_keypair() and setup_sender() methods no longer accept an explicit RNG argument, and the crate upgraded its bounds to rand_core 0.9. To maintain our deterministic ChaCha20Rng entropy source across all comparative benchmarks, this PR introduces a zero-cost RhChaCha20Rng reference wrapper and migrates all rust-hpke call sites to use the new _with_rng() variants. Notably, the wrapper natively delegates next_u32/next_u64 calls to prevent severe performance regressions observed during X25519 rejection sampling (e.g. local kem/x25519/rust_hpke/generate benchmarking — ~25.6 µs before vs. ~56.4 µs after). The six new rust-hpke rows cover kem/xwing (i.e. generate, derive_key_pair, encap_via_setup_sender, decap_via_setup_receiver functions) and xwing_chacha20 (i.e. setup_sender, setup_receiver functions). Previously, those groups benched hpke-ng and hpke-rs but omitted rust-hpke, making the comparative benchmarks suite incomplete. Note that all three libraries implement draft-connolly-cfrg-xwing-kem-06 (i.e. draft no. 6) via the same x-wing crate (^0.1.0-rc.0) and share KEM_ID 0x647a; thus, timing is directly comparable with no caveats.

2. The README performance section is updated to reflect that rust-hpke benchmarks now cover X-Wing, and benchmark totals as well as X-Wing-specific deltas are replaced with TBD pending a clean cargo bench run. In addition, the getrandom feature flag is removed from the hpke dependency in Cargo.toml. It is a default feature in rust-hpke and listing it explicitly is redundant.

All new bench groups compile and run under:

cargo bench --features comparative --bench comparative -- "kem/xwing"
cargo bench --features comparative --bench comparative -- "xwing_chacha20"