* Studio: UNSLOTH_NPM_REGISTRY opt-in for corporate npm mirrors (#6491)
studio/frontend/.npmrc pins registry=https://registry.npmjs.org/ as a
supply-chain lock. A project-level pin takes precedence over a user's
~/.npmrc, so behind a corporate firewall that blocks npmjs.org the
frontend bun/npm install hit npmjs.org directly and failed with 403.
Add an opt-in UNSLOTH_NPM_REGISTRY env var (off by default). When set it
is threaded as --registry into every registry-touching install in
setup.sh, setup.ps1 and build.sh (bun bootstrap, bun install + retry, npm
fallback, OXC validator runtime). --registry is the highest-precedence
override for both bun and npm and leaves min-release-age and save-exact
in force, so the default lock is unchanged for everyone else.
On an install failure that looks like a blocked registry, print guidance
pointing at UNSLOTH_NPM_REGISTRY and auto-suggest the mirror already set
in the user's npm config. Registries are never switched automatically.
Also correct the .npmrc comment: the pin does not block an ambient
NPM_CONFIG_REGISTRY env var (npm and bun honor that at higher precedence);
it only guards against a lower-precedence stale ~/.npmrc.
* Studio: make the registry hint reachable under set -e; clean temp log (#6491)
run_quiet_no_exit returns non-zero on failure, which under `set -euo
pipefail` exits the script at the call site before the exit code is
captured, so the new UNSLOTH_NPM_REGISTRY hint never printed on the npm
fallback and OXC validator paths. Guard both with `|| _rc=$?` (the same
idiom every other run_quiet_no_exit caller already uses) so the failure
branch runs, and remove the _FRONTEND_INSTALL_LOG temp file on the
early-exit path.
* Studio: detect the user's mirror outside the pinned frontend dir (#6491)
_suggest_npm_registry / Show-NpmRegistryHint run while the cwd is still
studio/frontend, whose .npmrc pins registry=https://registry.npmjs.org/.
So `npm config get registry` returned that pin instead of the user's
~/.npmrc mirror, and the "Detected a registry" branch was skipped for the
main corporate case (mirror set in ~/.npmrc). Run the lookup from a
directory with no project .npmrc (/ in bash, the temp dir in PowerShell)
so the user/global mirror is surfaced. The NPM_CONFIG_REGISTRY env check
is unchanged and still takes precedence.