refactor(runtime): isolate SPX interpreted run sessions

[joeykchen]

Summary

Extracts the driver-independent interpreted runtime foundation from #1753.

• Give spx run explicit project, asset, and disposable session roots without changing the parent process working directory.
• Prepare a session-local GDExtension scaffold and pass one canonical root environment to the Engine child.
• Make ispxnative pin the project filesystem for deferred resource loads while retaining bounded legacy asset compatibility.
• Configure Engine filesystem roots before interpreter initialization and publish a new resource schema only after a successful build.

This PR contains no Project Driver declaration, driverprotocol import, XGo dispatch code, runtime bundle/payload code, or driver terminology.

Behavior covered

• spx run --path <project> keeps the caller working directory unchanged.
• The Engine runs from <project>/.temp and receives explicit SPX_PROJECT_DIR, SPX_ASSET_DIR, and SPX_SESSION_DIR.
• Ambient Engine --path values are replaced by the isolated session root.
• Existing shared external-asset compatibility remains available only through the legacy SPX command path.
• Project filesystem handles remain valid for deferred Engine resource reads.

Verification

go test ./...
go test -race ./cmd/ispxnative ./cmd/spx/internal/command ./internal/engine ./internal/interpruntime ./internal/scaffold ./pkg/ispx
go test -tags=packmode ./internal/engine ./pkg/ispx
(cd cmd/ispx && go test ./...)
go vet ./cmd/ispxnative ./cmd/spx/internal/command ./internal/engine ./internal/interpruntime ./internal/scaffold ./pkg/ispx
python3 .github/scripts/runtime_lock_snapshot.py check
python3 .github/scripts/runtime_lock_snapshot.py verify-godot
python3 .github/scripts/release_bump_test.py
python3 .github/scripts/runtime_build_contract_test.py
GODOT_SRC=/Users/mac/qiniu/godot make doctor

A real tutorial/05-Animation smoke test also verified spx run and spx runnative session startup and clean child-process termination.

Relationship to #1753

This PR is independent of goplus/mod#165 and goplus/xgo#2849. Keep #1753 unchanged while this PR is reviewed. After merge, #1753 can rebase onto the updated dev branch and drop this shared layer from its diff.

Release metadata

This source change declares SPX v3.2.4 with Engine Runtime 2.4.4; runtime ABI remains 2. The immutable 2.4.4 lock snapshot is included. Since runtime-v2.4.4 is not published, CI resolves it as missing and validates the runtime from source. No release workflow was dispatched and nothing was published.

[fennoai]

Reviewed the complete fixed diff for the session-root isolation, Engine filesystem-root handling, native interpreter setup, scaffold generation, and release metadata contracts. I did not find a concrete correctness, compatibility, security, or reliability regression introduced by this PR.

Verification completed:

• go test ./...
• go test -tags=packmode ./...
• go test -race ./cmd/ispxnative ./cmd/spx/internal/command ./internal/engine ./internal/interpruntime ./internal/scaffold ./pkg/ispx
• go vet ./cmd/ispxnative ./cmd/spx/internal/command ./internal/engine ./internal/interpruntime ./internal/scaffold ./pkg/ispx
• Runtime lock, Godot ancestry, release bump, and runtime build-contract checks