Expand copilot-instructions: code style, module organization, and safety model

[Copilot]

The instructions were thin on three topics contributors need to reason about correctly: code style conventions, how modules are structured, and how client input is validated. Also corrects the false claim that there are no unit tests.

Code Style

• Documents glob import conventions (enums::*, error::*, PacketID::*) and when to use them
• Naming: Rust snake_case/PascalCase vs. FusionFall Hungarian notation on packet fields; #[allow(non_snake_case)] pattern
• Casting: infallible as for well-bounded protocol fields, try_into()? for enums or out-of-range values
• Immediately-invoked closure pattern for handlers that must produce SUCC or FAIL response packets

Module Organization

• Library (src/lib.rs) vs. binary (src/bin/) separation and what belongs where
• Private-module + pub use * re-export pattern (mod ffclient; pub use ffclient::*;)
• Global singletons via OnceLock/LazyLock (config_get(), tdata_get(), db_get()); mutable game state stays in ShardServerState/LoginServerState
• Handler files grouped by feature domain; file-local mod helpers { … } submodule at bottom of file

Safety — Trust-but-Verify

Documents the four validation layers every packet passes through:

1. Connection-level filtering (can_send_packet): unknown clients may only send a 3-packet whitelist; all others filtered by direction bitmask
2. Structural deserialization (pkt.get()): alignment + size checked before transmute; misaligned data returns FFError, not UB
3. Semantic validation in handlers: enum fields via try_into()?, slot bounds, target count caps, entity lookups returning FFResult, resource/currency checks, validate_perms() guard on all GM handlers
4. Session guard: serial-key handshake + pending_entering_uids prevent replay and double-enter races

Also documents why the single unsafe block in bytes_to_struct is sound and points to the net/mod.rs unit tests that cover it.

Unit Tests

Corrects the erroneous "no unit tests" statement. Tests exist in util, config, chunk, net/mod, net/crypto, entity/mod, and tabledata.