ci: default tests to latest, add pkg.pr.new dispatch input

[fengmk2]

Summary

Removes alpha-channel testing and replaces it with on-demand pkg.pr.new verification triggered manually.

• Dropped the version: [latest, alpha] matrix from every test job. Push / PR / merge_group runs now exercise only the latest published Vite+ (the action's default). Jobs whose only matrix dimension was version lost their strategy: block; jobs with other dimensions (os, node-version, lockfile) kept theirs.
• Added a workflow_dispatch input pr_version (a PR number or commit SHA, e.g. 1569) for verifying an unreleased Vite+ build on demand.

How pkg.pr.new testing works

pkg.pr.new builds install via VP_PR_VERSION (PR number / commit SHA), not the version input — the install script checks VP_PR_VERSION before VP_VERSION and uses it to pull the matching build from pkg.pr.new. The input feeds a workflow-level env var:

env:
  VP_PR_VERSION: ${{ github.event.inputs.pr_version }}

Since the action spreads ...process.env into the install, this flows straight through and wins even though the action sets VP_VERSION=latest. On every non-dispatch event the value is empty, which the script treats as unset → latest. So one manual dispatch verifies a candidate build across the entire matrix (install, node-version, cache, exec, sfw) with no source change to the action.

Notes

• The build job is pinned to latest (VP_PR_VERSION: "") so a pre-release bundler can't shift dist/ output and fail the "dist is up to date" diff for unrelated reasons.
• The sfw jobs now ride the dispatched version too; their stale "covers the alpha channel" comments were updated.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates CI to test the default latest Vite+ release on normal events while allowing manual workflow dispatches to verify an unreleased pkg.pr.new Vite+ build via VP_PR_VERSION.

Changes:

• Adds a workflow_dispatch pr_version input and workflow-level VP_PR_VERSION environment variable.
• Removes the latest/alpha version matrix from test jobs.
• Pins the build job to latest by clearing VP_PR_VERSION, and updates related sfw comments.

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