feat(xgodriver): implement SPX project-driver source mode

[joeykchen]

Summary

Supersedes goplus/spx#1742, which GitHub closed when its fork head branch was renamed.

This PR implements the SPX side of XGo Project Driver v1 described in #1741. It intentionally completes source mode only; published SPX module/bridge mode remains fail-closed until immutable bridge manifests and the coordinated release pipeline are available.

Project Driver integration

• Declare driver v1 github.com/goplus/spx/v3/cmd/xgodriver in gox.mod.
• Add the SPX driver under cmd/xgodriver and internal/xgodriver, using driverprotocol, DriverPackage, DriverOrigin, and the xgo-driver-v1 preamble.
• Support xgo run, xgo build, and xgo install when SPX is the main module, a workspace module, or an unversioned local replacement.
• Resolve project metadata, driver identity, and bridge source from one effective Go graph with declaration and module-provenance validation.
• Let the driver own driver-specific Go generation in a private work directory; v1 defines no XGo pre-generated artifact handoff.
• Reject published/versioned SPX bridge mode explicitly until immutable bridge manifests exist.

Engine runtime and packaging

Project Driver is separate from the true Engine runtime layer:

• Acquire Engine/PCK from a verified local runtime manifest or a code-pinned published release manifest for Engine Runtime 2.4.3 / ABI 2.
• Keep independent read-only project and asset roots plus a disposable session root for xgo run.
• Build a fresh interpreter bridge from the SPX source selected by the effective graph.
• Build an allowlisted, self-contained launcher containing Engine, PCK, bridge, manifests, and project payload.
• Reuse the transactional build path for xgo install.
• Validate file type, size, SHA-256, archive paths, collisions, identity stability, and cached components; repair damaged cache entries under a lease.
• Preserve Engine exit status and supervise cancellation, signals, and child-process cleanup.
• Allow a built launcher to run offline without the Go/XGo/SPX toolchain.

The bilingual design document is now:

• docs/en/dev/engine/xgo-project-driver-proposal-issue.md
• docs/zh/dev/engine/xgo-project-driver-proposal-issue.md

Verification

A local go.work containing the Mod, XGo, and SPX checkouts was used for coordinated verification.

go test -race ./internal/xgodriver ./cmd/xgodriver ./internal/runtimepayload ./x/xgolauncher
go test ./...
go vet ./...
GOWORK=off GODOT_SRC=/path/to/godot make buildctl

The local XGo binary also passed real tutorial/05-Animation smoke tests for:

• xgo run . -- -headless
• xgo build -o <output> ., followed by launching the executable
• GOBIN=<bin> xgo install ., followed by launching the installed executable

Process-chain inspection confirmed XGo -> xgo-driver-v1 -> SPX driver -> gdspxrt2.4.3, with no child processes left after cancellation.

Dependencies and release order

• Depends on goplus/mod#165 for driver metadata, driverprotocol, and resolved provenance.
• Depends on goplus/xgo#2849 for driver discovery, validation, process supervision, and transactional run/build/install dispatch.
• Before merge/release, SPX must consume released companion module versions.
• Published bridge mode remains follow-up work.

Part of #1741. This PR must not close the proposal while published mode and coordinated release remain pending.