feat(spx): add driver-independent buildlauncher

[joeykchen]

Summary

• add spx buildlauncher without starting Godot or requiring a local XGo driver
• freeze and verify Go module graph metadata across bridge and launcher builds
• protect project inputs with private staging and atomic output replacement
• document the standalone launcher workflow in English and Chinese

Built on the launchpack service merged in #1766.

Verification

• go test -count=1 -race ./...
• go vet ./cmd/spx/internal/command ./internal/launchpack
• Windows amd64 pure_engine cross-compile
• gofmt and git diff checks

[fennoai]

Reviewed the complete fixed diff for command dispatch, module-graph provenance, project metadata, output staging, platform-specific replacement, and the added tests/docs. The buildlauncher path is generally well-contained and the focused Linux package tests pass. One compatibility issue should be addressed before merging.

Validation: go test ./cmd/spx/internal/command ./internal/launchpack passed. A Windows cross-build was attempted but remains blocked by existing non-Windows package symbols in internal/gdengine.

[fennoai]

[P2] Scope the launcher output flag to buildlauncher

Registering -o in the shared flag set makes it valid for every command, but only buildlauncher consumes Args.Output; the legacy command paths ignore it (and ExtraArgs.String does not forward it). This changes previously rejected invocations such as spx run -o out into silently successful runs that discard the user's requested output, which can hide a typo or produce an unexpected artifact. Keep this flag scoped to buildlauncher, or explicitly reject it for the other command paths.