feat: add prepared write

[kvizyx]

Closes #546

Adds PreparedMessage and Conn.WritePrepared for writing the same message to many connections, such as when broadcasting. On connections that compress without context takeover, the message is compressed once and the result is shared instead of being compressed per connection.

Other connections use the regular write path: with context takeover each connection compresses against its own sliding window, so the result cannot be shared, and without compression there is nothing expensive to share. Only the compressed payload is cached so it works for both server and client connections, and clients still mask every write with a new key. On Wasm, WritePrepared is equivalent to Write.

Without context takeover, Write and WritePrepared now also send a message uncompressed if compression does not make it smaller, such as already compressed data, as RFC 7692 allows. With context takeover the message stays compressed to keep the peer's sliding window in sync (picked up this approach from uWebSockets).

Also adds a broadcast example in internal/examples/broadcast.

Benchmarks

BenchmarkBroadcast writes a 1 KiB message to 16 connections per op:

Benchmark	Write	WritePrepared	Delta
Server, no context takeover	53.5 µs, 3.2 KiB, 64 allocs	14.8 µs, 5.0 KiB, 70 allocs	-72.29%
Client, no context takeover	68.4 µs, 3.4 KiB, 64 allocs	28.6 µs, 5.2 KiB, 70 allocs	-58.25%
Server, context takeover	46.4 µs, 2.4 KiB, 64 allocs	45.0 µs, 3.5 KiB, 66 allocs	-2.94%
Client, context takeover	58.3 µs, 2.4 KiB, 64 allocs	59.4 µs, 3.5 KiB, 66 allocs	~
Server, compression disabled	11.8 µs, 2.4 KiB, 64 allocs	12.4 µs, 3.5 KiB, 66 allocs	+5.21%
Client, compression disabled	26.7 µs, 2.4 KiB, 64 allocs	26.6 µs, 3.5 KiB, 66 allocs	~

The extra allocations happen once per broadcast, not per connection: 2 for NewPreparedMessage (the struct and its copy of the message) + 4 for compressing the message once without context takeover.

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AI: Part of this changes were written with AI. I reviewed all of the code :)