refactor: Decompose ipc_server.rs into module directory

[2witstudios]

Summary

• Extract streaming protocol handlers (attach, grid, status) from ipc_server.rs into ipc_server/streams.rs, isolating the highest-churn code from server lifecycle and dispatch logic
• Move all 9 tests into ipc_server/tests.rs
• Server lifecycle, dispatch, and wire helpers remain in ipc_server/mod.rs
• Zero logic changes — purely structural extraction following the existing engine/ module pattern

Test plan

• cargo clippy -p pu-engine — clean, no warnings
• cargo test -p pu-engine --lib ipc_server — all 9 tests pass

🤖 Generated with Claude Code

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