Harden discovery metadata handling

[vikranthreddimasu]

Summary

• Validate discovered peer GPU, RAM, score, and transport-port metadata before creating peer records.
• Bound printable identity fields before retaining or displaying them.
• Trim rich network-link advertisements to DNS TXT's 255-byte item limit so mDNS registration cannot fail.

Security changes

• Reject negative, non-finite, oversized, and malformed untrusted mDNS metadata.
• Reject control characters in terminal-visible peer identity and hardware fields.
• No authentication, authorization, token, credential, TLS, or destructive-action behavior changed.

Usability changes

• Prevent unusable peers with invalid control/data port combinations from entering discovery.
• Preserve as much topology metadata as fits instead of failing node registration on large advertisements.
• Keep v2.1 data-port fallback behavior when the derived port is valid.

Tests/checks run

• python -m pytest tests/test_pool/test_discovery.py tests/test_pool/test_topology.py tests/test_security/test_fleet_discovery.py -q — 82 passed.
• make lint — ruff clean; mypy success across 55 source files.
• make test — 1104 passed, 3 skipped, 2 existing deprecation warnings.
• python -m pip wheel . --no-deps — built macfleet-2.2.1-py3-none-any.whl.
• git diff --check master...HEAD — clean.
• Diff-file sensitive-pattern scan — no matches.

Commits

• dda0573 Validate discovered peer metadata
• f73f607 Bound discovery identity fields
• e7b7c9b Keep discovery advertisements wire-safe

Risks / manual review notes

• Low-risk discovery validation change; no credential or authentication paths changed.
• Hardware limits are intentionally generous (1024 GPU cores, 65536 GB RAM) to reject corrupt metadata without constraining plausible future Macs.
• Oversized topology advertisements retain the largest ordered subset that fits one TXT item.

Merge status

• Merged into master after all 11 GitHub checks passed and the PR remained conflict-free.
• Merge commit: 7b7611efea36271bd7d3bea4f11dba42660e4d94.