The `/`, `/index.html`, and `/session/<id>` routes are the hottest
navigations. Each request re-read the ~190 KB static/index.html from disk
and re-applied two process-constant token substitutions
(__WEBUI_VERSION__, __MAX_UPLOAD_BYTES__), then ran inject_extension_tags()
(which itself reads the extension state + manifest from disk).
WEBUI_VERSION and MAX_UPLOAD_BYTES are fixed for the process lifetime, so
this caches the partially rendered template in an in-process cache keyed by
(size, mtime_ns) -- mirroring the existing _STATIC_CACHE pattern in this
file -- so a redeploy is still picked up without a restart. The per-session
CSRF token and runtime extension tags are still applied per request, so
output is byte-identical to before.
Verified: byte-identical to the old inline render, CSRF still varies per
request, cache invalidates on mtime change, ~5x faster shell render in a
local microbenchmark. Adds tests/test_index_shell_template_cache.py.
Rod Boev