Closes #435

### Problem

When compression was enabled, `Conn.Write` sent messages across many small frames due to the flate library's internal `bufferFlushSize` (240 bytes). Each flush triggered a `writeFrame` call, producing alternating ~236 and 4 byte frames. This broke clients like Unreal Engine that only process one frame per tick.

### Solution

`Conn.Write` now compresses the entire message into a buffer first, then transmits it as a single frame. Messages below `flateThreshold` bypass compression and are sent uncompressed in a single frame.

For `CompressionContextTakeover` mode, the flateWriter destination is restored after buffered compression to ensure subsequent `Writer()` streaming calls work correctly.

### Changes

- **write.go**: Buffer compressed output before transmission
- **compress_test.go**: Added regression tests for single-frame behavior and Write/Writer interop

- humanize some comments/remove lots of unneeded ones
- refactor write
- ensure we close?

- compression_test.go: add a `Writer` -> `Write` transition
- compression_test.go: Claude sucks at variable names apparently
- write.go: make sure to hold a lock 🤦
- write.go: combine two identical if branches
- write.go: rename `writeFull` because the name was bad

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>