refactor(core): remove out-of-scope HTTP/2 client/proxy, reorganize docs, refresh README

HttpProxy and Http2Client (719 LOC) shipped a reverse-proxy adapter and outbound HTTP/2
client from flash core with zero callers anywhere in the server itself — only each
other and their own tests. An HTTP/1.1+2 server framework has no business bundling an
outbound client; that capability belongs in its own flash-extensions/flash-ext-*
module if/when it's needed. Removed, along with the now-dead src/bench load driver
that depended on Http2Client (no replacement client written here — flagged as
follow-up work, not silently dropped).

docs/http2/ had accumulated core, cross-protocol documentation alongside genuine
HTTP/2-protocol internals: HTTP1-HARDENING, TRANSPORT, MESSAGE-MODEL,
TRAILERS-AND-STREAMING and BYTES all describe machinery HTTP/1.1 and HTTP/2 share, not
HTTP/2 specifically. Moved to a new docs/core/, leaving docs/http2/ to the protocol
layers, wire internals and operational docs that are actually HTTP/2-specific.
CLEARTEXT-AND-PROXY.md renamed to CLEARTEXT.md and its now-removed upstream-client
section cut, matching the source removal above.

README.md: removed the "HTTP/2 upstream proxy" section (documented the deleted
HttpProxy/Http2Client), the flash-bench module row and build command (not a module
that exists in this repo), and fixed every doc link to the new docs/core/ paths.
Added the new FlashConfiguration.maxConnections field to the configuration reference.

src/bench/ (a load-test harness distinct from the JMH suite, not wired into any Maven
profile or CI) is committed here for the first time.

flash/docs/http2/README.md

@@ -18,20 +18,18 @@ listener / TLS
         <- HTTP/2 stream writer ----+
 ```
 
-## Start here
+This page covers the HTTP/2-specific layers only. The transport, message model, and byte
+primitives shared with HTTP/1.1 live in [`../core/`](../core/README.md).
+
+## Protocol layers
 
-- [HTTP/1.1 hardening](HTTP1-HARDENING.md) — message-boundary rules, timeouts and negotiation.
-- [Transport](TRANSPORT.md) — listeners, connection ownership, TLS and virtual threads.
-- [Message model](MESSAGE-MODEL.md) — shared request/response objects and their lifetime contract.
 - [Connection](CONNECTION.md) and [streams](STREAMS.md) — HTTP/2 connection and stream state.
 - [Flow control](FLOW-CONTROL.md) — request backpressure and streamed responses.
-- [Trailers and streaming](TRAILERS-AND-STREAMING.md) — the public cross-protocol APIs.
-- [Cleartext and proxying](CLEARTEXT-AND-PROXY.md) — prior knowledge and the upstream h2 client.
+- [Cleartext](CLEARTEXT.md) — prior knowledge and the 421 misdirected-request rule.
 - [WebSockets](WEBSOCKET.md) — RFC 8441 extended CONNECT using the existing WebSocket API.
 
 ## Wire internals
 
-- [Byte primitives](BYTES.md) — reusable views, scanning and bounded slice lifetimes.
 - [Serialized writer](WRITER.md) — the single-owner output path and contention model.
 - [Frames](FRAMES.md) — frame parsing, validation and error scope.
 - [HPACK](HPACK.md) — integer/Huffman coding and static/dynamic table ownership.
@@ -43,9 +41,3 @@ listener / TLS
 - [Compliance](COMPLIANCE.md) — h2spec, interoperability, fuzzing and deliberate omissions.
 - [Performance](PERFORMANCE.md) and [CI baselines](BASELINES.md) — measurements and regression
   gates, including the comparison with nghttpd.
-
-## Design history
-
-[Decisions](DECISIONS.md) records non-obvious trade-offs and rejected alternatives. The
-implementation plan is retained as historical engineering evidence; it is not required to use or
-extend the runtime.