Update crypto dependencies to chacha20poly1305 0.11 and dalek v3

[Mathieu Morrissette (mmorrissette-devolutions)]

• chacha20poly1305 0.10 -> 0.11, ed25519-dalek 2 -> 3, x25519-dalek 2 -> 3, STREAM now from the aead-stream crate (removed from aead 0.6)
• private keys are now generated directly from the OS CSPRNG
• zeroize hardening: cipher key schedule (chacha20poly1305/zeroize), secret shares (blahaj/zeroize_memory), serialization intermediates
• online_ciphertext doctests now use the STREAM API
• added a cross-version STREAM conformity vector

Wire formats are unchanged and covered by conformity vectors. Note: dalek v3 types appear in public From impls and MSRV rises to 1.85, so the Rust API is semver-breaking.

[Copilot]

Pull request overview

This PR upgrades the cryptographic dependency stack (chacha20poly1305 0.10→0.11, ed25519-dalek 2→3, x25519-dalek 2→3) and migrates the STREAM construction to the newly split aead-stream crate, since stream was removed from aead 0.6. Alongside the version bumps, it hardens secret handling with zeroize and moves private-key generation to draw seeds directly from the OS CSPRNG (SysRng) via a new utils::random_bytes helper. As the author notes, dalek v3 types surface in public From impls and the MSRV rises, so the Rust API is semver-breaking, but the on-disk/wire formats are unchanged and covered by conformity vectors.

Changes:

• Bumped crypto crates and switched STREAM to aead-stream; updated cipher construction to new_from_slice(...) and error/From plumbing to chacha20poly1305::aead::Error.
• Added utils::random_bytes::<N>() -> Zeroizing<[u8; N]> and routed all private-key/ephemeral-key generation through it (plus additional Zeroizing of serialization intermediates and secret shares).
• Rewrote the online_ciphertext module docs/doctests to reflect the chunked STREAM API and added a cross-version STREAM known-answer conformity vector.

Reviewed changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Cargo.toml	Bumps crypto deps, adds aead-stream, and enables zeroize-related features.
Cargo.lock	Regenerated lock reflecting the new dependency graph (single aead 0.6).
src/utils.rs	Adds random_bytes helper returning a zeroizing fixed-size buffer from SysRng.
src/key/key_v1.rs	Generates/loads X25519 secrets via random_bytes and zeroizes intermediates.
src/key/secret_key_v1.rs	Uses random_bytes for generation; zeroizes the TryFrom buffer.
src/signing_key/signing_key_v1.rs	Builds the Ed25519 key from OS-seeded bytes; zeroizes keypair serialization.
src/secret_sharing/secret_sharing_v1.rs	Zeroizes share-serialization intermediates; updates the zeroize rationale comment.
src/ciphertext/ciphertext_v2.rs	Switches to new_from_slice and owned XNonce for the 0.11 API.
src/online_ciphertext/online_ciphertext_v1.rs	Migrates STREAM types to aead-stream and OS-seeded ephemeral keys.
src/online_ciphertext/mod.rs	Rewrites module docs/doctests to the chunked STREAM API with valid intra-doc links.
src/error.rs	Retargets the AEAD error From impl to chacha20poly1305::aead::Error.
tests/conformity.rs	Adds a cross-version STREAM known-answer decryption vector.

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