Play Framework

fix: http.port from conf wins; plugin only overrides when explicitly set (PF-90)

Surfaced during Phase 7 when the user noticed http.port was declared in
both build.gradle.kts (httpPort.set(9000)) AND conf/application.conf
(http.port=9009 for payments, 9001 for api, 9000 for jclaw).

The plugin's httpPort had a convention default of 9000, so even apps
that customized http.port in conf would get silently overridden. payments
was listening on 9000 instead of 9009 under gradle playRun.

Fix:
- Drop httpPort.convention(9000) from Play1Extension. The property is
  now optional (no default value) — conf/application.conf decides
  when nothing's set in DSL or via -PhttpPort.
- registerPlayJvmTask: only adds --http.port arg when ext.httpPort
  is present (DSL set or -PhttpPort).
- spawnPlay (used by playStart/playRestart): now takes Int? for
  httpPort; only adds --http.port to the JVM cmd when non-null.
- PlayAutotestTask: needs to know the port for FirePhoque's
  application.url. Falls back to reading conf/application.conf's
  http.port (then framework default 9000) when ext.httpPort isn't set.
- PlayStatusTask already had this fallback pattern.
- Scaffold (playNewApp): no longer emits httpPort.set(9000) — it was
  redundant with the application-skel's commented-out http.port line.

Verified against payments (conf 9009): listens on 9009, not 9000.
Verified against api (conf 9001): listens on 9001. -PhttpPort=8765
override still works (per the afterEvaluate plumbing from previous
commit).