peer_id: rewrite parsing and error contracts

[uink45]

Summary

• rewrote src/peer_id/peer_id.c into helper-driven, bounds-checked parsing/formatting paths for legacy base58 and CIDv1 peer IDs
• aligned peer_id_to_string() with its documented contract by returning negative error codes on failure and deterministic output reset behavior
• removed mutation of caller-owned private-key input buffers in RSA/Ed25519/ECDSA helper flows
• replaced tests/peer_id/test_peer_id.c with vector-driven round-trip coverage plus explicit negative/boundary regression checks
• added Step 3/7 standards mapping and remediation evidence in standards/rewrite-pilot-6-peer-id.md

Validation

• cmake --build --preset macos-full --target peer_id peer_id_proto peer_id_rsa peer_id_ecdsa peer_id_ed25519 test_peer_id test_protocol_noise
• ctest --preset macos-full -R '^(Testpeer_id|Testprotocol_noise)$' --output-on-failure
• ctest --preset macos-full --output-on-failure
• cmake --preset linux-asan-ubsan
• cmake --build --preset linux-asan-ubsan --target test_peer_id test_protocol_noise
• ctest --preset linux-asan-ubsan -R '^(Testpeer_id|Testprotocol_noise)$' --output-on-failure
• ctest --preset linux-asan-ubsan --output-on-failure
• clang-format --style=file --dry-run --Werror src/peer_id/peer_id.c src/peer_id/peer_id_rsa.c src/peer_id/peer_id_ecdsa.c src/peer_id/peer_id_ed25519.c tests/peer_id/test_peer_id.c
• cppcheck --error-exitcode=1 --std=c99 --enable=warning,portability --quiet --suppress=missingIncludeSystem src/multiformats include/multiformats tests/multiformats src/peer_id include/peer_id tests/peer_id
• scripts/dev/run-fast-lane-local.sh --platform macos