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/WRITER.md

@@ -137,8 +137,8 @@ from the path this document's gate criteria are strictest about.
 ## Benchmark methodology
 
 `flash/src/jmh/java/dev/relism/flash/http2/frame/FrameWriterBenchmark.java` (a JMH source root
-registered only under the `jmh` Maven profile — see `DECISIONS.md`, `DEC-17`, for why it does not
-live in `src/test/java`) compares four harnesses at `threads` ∈ {1, 2, 4, 8, 16, 64}:
+registered only under the `jmh` Maven profile, not `src/test/java`) compares four harnesses at
+`threads` ∈ {1, 2, 4, 8, 16, 64}:
 
 - `trylock_mpsc` — the shipped `Http2FrameWriter` design.
 - `plain_lock` — every write blocks on `ReentrantLock.lock()` unconditionally (candidate (a)).
@@ -258,7 +258,7 @@ design's exclusive use of `ReentrantLock` (never `synchronized`) on every path t
 | 4 | Stress test green at every N, 1000 iterations, incl. parallelism=1 | 10 000/10 000 | **PASS** |
 
 **All four gate criteria are met.** `Http2FrameWriter` ships as designed: a `tryLock()` fast path
-with an intrusive MPSC fallback. See `DECISIONS.md` for the retained alternatives and evidence.
+with an intrusive MPSC fallback.
 
 ## What this design costs vs. what it saves