fix(test): add transport keypair uniqueness verification

[sanity]

Problem

CI occasionally fails on test_three_node_network_connectivity due to duplicate transport keypairs being generated for different nodes. When two nodes have the same public key, routing breaks and PUT operations time out.

Evidence from CI run 19970986958:

• Gateway key: v6MWKgqJxtAjv5xt
• Peer1 key: v6MWKgqK9qaZcuaY
• Peer2 key: v6MWKgqK9qaZcuaY ← Same as peer1!

The test shows all nodes connecting successfully (full mesh with 2 connections each), but PUT operations hang because routing fails when two nodes share the same identity.

The issue is intermittent - the test passes consistently locally and passed on CI re-run.

Root Cause Investigation

TransportKeypair::new() uses OsRng which should give unique keys each time. However, in CI environments under load, RNG behavior can sometimes produce collisions due to:

• Entropy exhaustion
• Timing issues
• Unknown environmental factors on self-hosted runners

This Solution

Generate all transport keypairs upfront at the start of each test and verify they're unique before proceeding:

1. Early detection: Catches the rare RNG collision immediately with a clear, actionable error message
2. Centralized generation: Uses pre-generated keys from a vector instead of inline generation
3. Zero overhead for passing tests: The uniqueness check is a fast HashSet operation

The fix is in the #[freenet_test] macro codegen, so all tests using the macro benefit from this protection.

Testing

• cargo test --package freenet --test connectivity passes (4 tests)
• cargo test --package freenet --test operations passes (9 tests)
• All pre-commit checks pass (fmt, clippy)

[AI-assisted - Claude]