feat(be): verify a browser's key and its announced successor

[sea-snake]

Design: #4224. Overview: #4230. The verifier only — its caller is the sign-in ceremony in #4268, which also removes the #[allow(dead_code)] this carries.

A session records which browser created it, so the user can see where they are signed in and end one browser's access everywhere at once. That is only worth anything if the browser named in a sign-in is the browser that made it; otherwise an attacker holding an access method could attribute their session to a browser the user recognises, and the settings list would lie. Ingress messages are public, so a key presented in one sign-in can be read off the wire by anyone.

verify_device_keys checks two signatures before a sign-in may name a browser: the current key over the session key and the announced successor, and the successor over the session key and the current key. Each is made under its own domain separator, ii-session-device-key and ii-session-device-successor, so neither can be replayed in the other's role.

Requiring possession of the successor is what closes the wire-reading attack: announcing a key you do not hold is impossible without its private half, so a key read off the wire cannot be claimed when its owner next presents one.

P-256 only, no fallback. A key that does not parse, a signature of the wrong length, and an empty signature are each refused rather than skipped.

Tests: device_key::tests (10), one accepting case and nine refusals — including a successor signature replayed as the current one, a signature over another session key, and one paired with a substituted successor.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to a7b0d9e.

Security Overview

• 🔎 Scanned files: 6 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/6d3e3b61-710b-4b2b-9c0b-43458b6314b6?scanId=002b1646-954e-4029-ac51-09fa049df36b&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► src/internet_identity/src/sessions.rs       Add new module for sessions ► src/internet_identity/src/sessions/browser_key.rs       Add browser key verification implementation and tests
Enhancement	► src/internet_identity/src/main.rs       Register new module sessions (mod sessions)
Enhancement	► src/canister_tests/Cargo.toml       Enable p256 workspace for tests
Enhancement	► Cargo.toml       Update p256 dependency to include pkcs8 feature