Hardened WebSocket transport and permessage-deflate for RFC 6455/7692 conformance

Fixed the client permessage-deflate encoder, which stripped the flush trailer on every
fragment and could truncate a SYNC_FLUSH, so a compressed message split across frames now
decodes, and an empty final fragment is encoded as a single 0x00 octet; the frame writer
derives the masking key from SecureRandom. The client no longer offers client_max_window_bits,
since the JDK Deflater cannot honour a reduced window, and rejects a handshake response that
echoes it. The session engine fails the connection on RSV1 set on a control frame, no longer
delivers a truncated message after a 1009 close, and surfaces a selection key cancelled during
shutdown as an I/O error on write so the engine takes its close path instead of retaining the
frame as back-pressure.

Extension negotiation on the server is stricter. Offers are parsed from every
Sec-WebSocket-Extensions field and combined as RFC 6455 requires, so a client's fallback offer
in a later field is still examined; a parameter repeated within an offer drops that offer; and
the permessage-deflate factory accepts only recognised parameters with well-formed values, the
no-context-takeover parameters carrying no value and the window-bits parameters being exactly
15, the only size the JDK Deflater supports. A given extension name is accepted only once, and
an unacceptable offer is declined rather than failing the handshake.

On the HTTP/2 server the handler advertises a bounded inbound flow-control window and bounds
the outbound queue by a byte budget, splitting large writes into chunks and signalling the
reactor after each so a write larger than the budget cannot deadlock; the frame reader rejects
a 64-bit length with the most significant bit set, the frame writer enforces the 125-byte
control frame limit and truncates the CLOSE reason, and the processor fails a data frame
received in the middle of a fragmented message. The HTTP/2 client accepts any 2xx handshake
response.

Both permessage-deflate implementations release their Deflater and Inflater when the session
ends. The extension chains close every codec they hold even if one of them fails, the
negotiation closes each extension exactly once, and the session engine coordinates the encoder
and decoder close with try/finally so a failure in one still releases the other.