refactor: consolidate play packet routing into a declarative descriptor table

[carabistouflette]

Type of change

• Block implementation
• Item implementation
• Command implementation
• Entity implementation
• Bug fix
• New feature
• Breaking change
• Refactor / code cleanup
• Performance improvement

Description

Consolidates the five dispatch sites for scheduled serverbound play packets — decode_play_packet, execution(), can_process_before_join, can_process_during_domain_handshake, and handle() — into a single declarative PLAY_PACKET_DESCRIPTORS table of PlayPacketDescriptor entries (id, decode fn, execution class, join gates, handler fn). Previously these lived in five hand-maintained matches that could drift independently (e.g. adding a packet to decode but forgetting its concurrency class).

• ScheduledPlayPacket carries its resolved descriptor, so execution class, gates, and handler dispatch are single lookups resolved once at decode time.
• Per-action concurrency (PlayerCommand, PlayerAction) is preserved through per-descriptor execution functions with their original audit comments.
• Immediate packets (keep alive, ping request, chunk batch received) keep their dedicated decode path; the join-gate id checks fall back to explicit immediate ids so pre-join/during-handshake behavior is unchanged.
• decode_play_packet shrinks to an immediate-packet match plus one descriptor lookup.

How this was tested

• All classification, routing, and domain-gate tests preserved and passing: cargo test -p steel-core --lib connection::java (15 tests) and cargo test -p steel-core --lib (2422 tests); cargo clippy -r --all-targets clean.
• Dispatch cost is one linear scan over 37 ids per decode (vs the previous match) — dominated by payload decode I/O, measured as noise; no behavioral change intended.

Screenshots / logs

N/A.

Checklist

• Code builds w/o errors or warnings
• Self-reviewed the diff
• Docs updated (if applicable) — N/A
• No leftover debug code / comments

Additional notes

• Contingency from the plan: if binary size or fn-pointer overhead is flagged, the table can be macro-generated instead.
• Env: Linux, Rust nightly, Minecraft protocol 776 (0.15.2+mc26.2).