Play Framework

chore(build,test): silence ant warnings + finish APP_SECRET→PLAY_SECRET rename

Two unrelated cleanups bundled because both are small and surfaced by a single
ant test run.

== Ant build warnings ==

modules/{testrunner,docviewer}/build.xml gain includeantruntime="false" on
their javac elements, silencing the "'includeantruntime' was not set"
warning. Neither module imports Ant tasks so the runtime classpath is
unnecessary; the framework's own build.xml needs includeantruntime="true" for
play.ant.* and is left alone.

== APP_SECRET → PLAY_SECRET ==

The canonical environment variable name for the application secret is
PLAY_SECRET (set in framework/pym/play/utils.py:DEFAULT_SECRET_VAR, written
to .env by `play secret`, used in resources/application-skel and the
integration testapp). Three stale APP_SECRET references remained:

  framework/src/play/Play.java
    Error message in the secret-validation path now points users at PLAY_SECRET
    via env var or -DPLAY_SECRET=... JVM flag.

  framework/pym/play/application.py
    Comment in PlayApplication.__init__ documenting how .env loads variables
    now references PLAY_SECRET.

  framework/test-src/play/EnvVarConfigTest.java
    Constant APP_SECRET_KEY → PLAY_SECRET_KEY; field savedAppSecret →
    savedPlaySecret; placeholder ${APP_SECRET} → ${PLAY_SECRET} in writeConfig.
    Rationale aside: setUp now sets the system property so the test config's
    required ${PLAY_SECRET} placeholder resolves cleanly, eliminating 9
    "WARNING: Cannot replace … in configuration" log lines per ant test run.
    The single remaining warning (PLAY_TEST_NONEXISTENT_VAR_12345) comes from
    noDefaultAndNoValueLeavesPlaceholder, which deliberately exercises the
    unresolved-placeholder code path.

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