Play Framework

docs: document virtual-thread opt-in toggles in CLAUDE.md

Adds an Architecture subsection covering VirtualThreadConfig and the four
play.threads.virtual.* properties (global plus per-subsystem invoker/jobs/mail
overrides). Also notes the property-prefix rename from the original
play.virtualThreads.* (commit 62d0d83, Feb 2026) to play.threads.virtual.*,
since the old prefix is silently ignored if anyone still has it in
application.conf.

Surfaces three load-bearing facts that are not obvious from reading the
code: virtual threads are off by default; the Netty event loop is
unchanged; the per-subsystem flags fall back to the global one when
unset. This prevents future readers (and AI assistants) from
overclaiming that the fork delivers virtual-thread throughput out of the
box.

Wire-up references: Invoker.java:410, JobsPlugin.java:199, Mail.java:253.

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