Play Framework

chore(docs,specs): retire stale VT toggle + pool-size references

The VT migration removed play.threads.virtual*, play.pool, and play.jobs.pool
as live knobs, but the user manual (EN+JA), 1.12.0 release notes, application
skeleton, and the active OpenSpec spec still described them as functional.
Drops the dead config sections from documentation/manual/configuration.textile
and documentation/manual_ja/configuration.textile, removes the commented
template lines from resources/application-skel/conf/application.conf, updates
the 1.12.0 Threading-model paragraph to note the toggle was removed in
subsequent 1.12.x patches (PF-53 historical bullet preserved), and rewrites
openspec/specs/virtual-threads/spec.md from its leftover delta-spec form into
an active baseline describing unconditional VT execution plus the warn-on-
retired-keys behavior.

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