feat(ispxnative): load pinned portable project config

[joeykchen]

Summary

• carry an optional, identity-bound portable project config through interpreted runtime roots
• load the config from a pinned session directory and reject path or content drift
• overlay the captured config without changing legacy run behavior when absent

Validation

• go test -race ./cmd/ispxnative ./internal/interpruntime
• go test ./...
• go vet ./...
• go test -tags pure_engine ./cmd/ispxnative ./internal/interpruntime
• Windows amd64 pure_engine compile
• go mod tidy -diff
• git diff --check

[fennoai]

Reviewed the fixed range 7b73af9d73edfeda5a5ee7c33e57e0b78f29ccf9...2a0efad9de3828db93d1cfb6f13c1268a38bacdd} across the runtime environment propagation, pinned config loading, and overlay filesystem units. The pinned snapshot and overlay behavior are covered by focused tests; go test -race ./cmd/ispxnative ./internal/interpruntime and git diff --check pass. One regression in forwarding the new portable-config contract is called out inline.

Additional findings

• internal/interpruntime/roots.go:166: [P1] Preserve portable config variables when building child environments: rootEnvKey now classifies SPX_PORTABLE_CONFIG_DIR and SPX_PORTABLE_CONFIG_IDENTITY as root variables, so this loop removes both from base, but the return value only re-adds the three legacy roots. Any caller that uses Roots.Environment to launch ispxnative therefore drops the pinned config directory and identity; the child sees no portable overlay and falls back to its project .config, defeating the new identity-bound behavior (or silently losing the portable config when the project copy differs). Preserve the validated pair when present, or filter only the three legacy keys here.