fix(rust): ed25519-dalek 3.0 removed the std feature — use alloc

PR #37 bumps x25519-dalek 2.0 -> 3.0 and ed25519-dalek 2.1 -> 3.0, and cargo
could not even resolve the dependency graph:

  error: failed to select a version for `ed25519-dalek`.
  package `pqf-reader` depends on `ed25519-dalek` with feature `std`
    but `ed25519-dalek` does not have that feature.

ed25519-dalek 3.0 went no_std-first and dropped `std`, which in 2.1.1 was
["alloc", "ed25519/std", "serde?/std", "sha2/std"]. Its default is now
["fast", "zeroize"], and `alloc` is the surviving equivalent of what this crate
needs. x25519-dalek 3.0 still has `static_secrets`, so that line was already
correct.

Because pqf-writer path-depends on pqf-reader, nothing Rust-side resolved, which
is why all 11 Rust-dependent checks failed while every .NET check passed.

Verified locally against the real toolchain, not by inspection:

  cargo build --release (pqf-reader)                    clean
  cargo build --release (pqf-writer)                    clean
  cargo test --release  (pqf-writer)                    12 passed, 0 failed
  cargo run --bin pqf-conformance                       pass=47 fail=0 skipped=0
  cargo build --target wasm32-unknown-unknown (bindings/wasm)  clean

The conformance run is the one that matters: it includes
TV-NEG-011 (refused: SignatureVerificationFailure), so Ed25519 verification still
behaves correctly under dalek 3.0 and curve25519-dalek 5.0, and all 47 vectors
agree with the .NET implementation.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: systemslibrarian

impl/rust/pqf-reader/Cargo.toml

@@ -28,7 +28,7 @@ x25519-dalek = { version = "3.0", features = ["static_secrets"] }
 ml-kem = "0.3.0"
 
 # Hybrid signatures
-ed25519-dalek = { version = "3.0", features = ["std"] }
+ed25519-dalek = { version = "3.0", features = ["alloc"] }
 # ml-dsa 0.1 routes the `signature::Verifier::verify` trait method to
 # FIPS 204 Algorithm 8 (no domain separator). BouncyCastle's pure
 # ML-DSA-87 signs with Algorithm 3 (with the empty-context prefix), so