ci: pin the Windows image and make its leg measurable

[martin-fleck-at]

Baseline

• Pin windows-2025 instead of the windows-latest alias, which advances a major Windows Server release under the workflow; a leg whose runtime and failure rate move on someone else's schedule cannot be measured against a change made here
• Bump actions/checkout to v7.0.1 and actions/setup-node to v7.0.0 across both workflows, so neither is forced onto a Node it does not declare
• Say what the Windows leg is actually for: child-process startup and per-file cost, alongside the path and URI divergence it already named
• Correct the claim that setup-node ignores packageManager; it reads the field to pick a cache, and still never installs that version

Dispatch knobs

• Add a turbo_concurrency input wired into the gate job's environment, so repeated runs of one commit can vary the cap without a commit each
• Add a skip_e2e input, since repeating a run to measure the gate should not also repeat four minutes e2e shares nothing with
• Leave both inert outside a dispatch: the input is empty on a push and a pull request, turbo reads empty as unset, and the e2e condition is written so only an explicit true can suppress the job

Two facts checked rather than assumed.

• turbo does read TURBO_CONCURRENCY, an invalid value being rejected by name, and does treat an empty one as absent
• Of the breaking changes across the three majors, the two reaching here are benign: setup-node's automatic caching converges on the cache the explicit input already selects, and its dropped dummy NODE_AUTH_TOKEN only helps a release authenticating over OIDC