Harden peer metadata validation

[vikranthreddimasu]

Summary of improvements

• Validates signed hardware-profile handshake JSON before constructing HardwareExchange.
• Bounds and sanitizes mDNS network topology advertisements.
• Rejects invalid discovered-node ports and non-finite/negative open-mode compute scores.
• Preserves explicit zero topology metrics while rejecting boolean telemetry.

Security changes

• Authenticated handshake peers can no longer send wrong-typed or out-of-range hardware fields that poison scheduler/election inputs after HMAC verification.
• mDNS-discovered peer metadata now rejects invalid ports and non-finite scores before entering discovery caches.
• No credentials, tokens, or secret files added; diff-only secret scan returned no matches.

Usability changes

• Malformed peer advertisements fail early and deterministically instead of surfacing later as bad routing/scoring behavior.
• Topology parsing still ignores unknown fields for forward compatibility.

Tests/checks run

• python -m pytest tests/test_security/test_hw_exchange.py -q (red first: 17 failures, then 41 passed)
• python -m pytest tests/test_pool/test_topology.py -q (red first for new cases, then 16 passed)
• python -m pytest tests/test_security/test_fleet_discovery.py -q (red first: 7 failures, then 24 passed)
• python -m pytest tests/test_security/test_hw_exchange.py tests/test_fuzz/test_fuzz_hw_exchange.py -q (51 passed)
• python -m pytest tests/test_pool/test_topology.py tests/test_pool/test_network.py tests/test_pool/test_registry.py -q (37 passed)
• python -m pytest tests/test_security/test_fleet_discovery.py tests/test_pool/test_peer_removed.py tests/test_pool/test_registry.py -q (37 passed)
• make lint (ruff passed; mypy passed with existing quantize.py note)
• python -m pytest tests/ -q (1021 passed, 2 expected deprecation warnings)
• git diff --check master...HEAD
• Diff-only secret scan with rg

Commit list

• d6520f6 Harden hardware profile handshake validation
• 805dfd8 Bound network topology advertisements
• b7ddf26 Validate discovered node metadata
• 9dedd56 Preserve zero topology metrics

Risks or manual review notes

• Manual review recommended before merge because this touches authenticated handshake and discovery metadata validation paths.
• Compatibility note: invalid advertised data_port/port records now reject instead of being cached.

Whether the PR was merged

• Not merged by automation; left open for manual review due to security-sensitive handshake/discovery changes.