Play Framework

[PF-34/PF-35/PF-36] Audit ship-blockers + 1.12.0 version bump + release notes

Three ship-blocker fixes from the post-PF-33 multi-agent audit, plus the
v1.12.0 release notes and the version bump in framework/build.xml.

PF-34: WebSocket Outbound close()/futureClose() data race
- The writeAndClose listener fires on the EventLoop and called futureClose()
  without holding the `this` monitor, racing with close()'s synchronized write
  to closeTask. Under JMM rules the listener could observe stale null
  closeTask and silently skip invoking the close promise, leaving the
  WebSocket connection orphaned.
- futureClose() is now synchronized; the onRedeem callback delegates to a new
  synchronized finalizeClose() helper instead of inline mutating closeTask.
  Java synchronized methods lock on the instance monitor; reentrant on the
  inline-callback case.
- Two reviewers (refcount and concurrency) independently flagged this —
  strongest possible signal for a real bug.

PF-35: serve500 leaked a ByteBuf on every 500 (3 sites)
- Each serve500 path did `ByteBuf buf = Unpooled.copiedBuffer(bytes);
  nettyResponse.content().clear().writeBytes(buf);` — the intermediate `buf`
  has refcount 1 and goes out of scope unreleased. Heap-backed so no direct-
  memory leak, but Netty's PARANOID leak detector flags this once a 500 is
  triggered.
- Replaced with the byte[] overload of writeBytes — drops the intermediate
  buffer entirely. Three sites: primary template render, static fallback
  render, last-ditch render.

PF-36: HEAD requests on error paths still sent a body (RFC 7230 §3.3.2)
- DefaultFullHttpResponse(version, status, buf) carries body bytes regardless
  of request method. The main writeResponse path handles HEAD correctly via
  byte[0] content; serve404, serve500 (3 sites), and the serveStatic error
  fallback did not.
- Now check `nettyRequest.method().equals(HttpMethod.HEAD)` at each error-path
  site. For HEAD: pass Unpooled.EMPTY_BUFFER as response content while
  preserving Content-Length (per RFC 7230 §3.3.2 second sentence — HEAD CL
  matches what the equivalent GET would return).

Release notes (documentation/manual/releases/release1.12.x/releasenotes-1.12.0.textile):
- Documents the breaking changes (removed deprecated ctors with migration
  paths, pipeline FQCN updates, maxContentLength semantic change)
- Inventories the new play.netty.spoolThresholdBytes config with tuning
  guidance
- Lists known limitations carrying forward (PF-37 through PF-43)
- Includes an upgrade checklist

build.xml: baseversion bumped 1.11.12 → 1.12.0 to match the v1.12.x branch
designation; ant package now produces play-1.12.0.zip.

Verified with `ant clean unittest`: 295/295 tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>