[nico2525nn]

Summary

• Align the implemented defaults with Minecraft Java 1.21.4: online authentication and secure-profile enforcement enabled, view distance 10, vanilla MOTD and easy difficulty, and a random seed for new worlds while preserving an existing level.dat seed.
• Match supported Java Properties behavior for separators, escapes, continuations, key case, UTF-8/ISO-8859-1 fallback, Boolean.parseBoolean, leading-plus integers, and difficulty IDs 0–3.
• Recognize canonical white-list while retaining whitelist as a legacy alias; align secure-chat advertisement and policy handling. Offline fake-client launchers explicitly disable authentication checks.
• Fix malformed doubled-sign integer parsing (+-N) and add regression coverage. Add a repo-wide conditional PR merge policy to AGENTS.md.

Validation

• RelWithDebInfo Ninja configure/build: PASS.
• Current head focused tests: test_properties 64/64, test_settings_matrix 49/49, test_secure_chat_policy 12/12; security guards pass.
• Full non-nightly CTest: 55/55 on previous head 5eb101d1 (516.24s). Exact-head GitHub Actions is running for b76c93b6ca294c311331067a18d70a1b731adee4; the prior run is not exact-head evidence.
• Exploratory script failures reproduced against the parent and documented in docs/VERIFICATION.md; no account-backed end-to-end login was run.

Scope and limitations

• This implements a subset, not complete server.properties parity. enforce-whitelist, query/status/network, permission, datapack, and several world/pack/operations properties remain unsupported or incomplete.
• ServerProperties::save is not equivalent to Java Properties.store; unsigned chat after a player session remains unverified.
• This PR is stacked on PR fix: align protocol primitives and stabilize smoke coverage #1 (chore/goal-cleanup-next) and targets that base only.