build: add Phase 0 workspace scaffolding

[snkmcb]

Summary

• add the Phase 0 root CMake and OpenStrata workspace
• scaffold installable physicsCore and physicsJolt packages under the usd_physics public boundary
• add dependency-boundary, documentation-link, version-coherence, and clean-prefix consumer tests
• record the Stage Runner extraction baseline and synchronize architecture, capability, roadmap, and verification docs
• generate the digest-pinned Windows/Linux OpenStrata CI workflow

Review fixes

• validate both public and private CMake link edges
• reject version drift between VERSION, openstrata.toml, and library manifests
• keep documentation and boundary scans out of generated build/runtime trees
• remove whitespace errors reported by git diff --check

Verification

• ost build on Windows with the pinned cy2026 / OpenUSD 26.08 runtime
• ost test: 6/6 passed
• ost library test: 1/1 passed for both libraries
• ost library verify-consumer: passed for both installed packages
• ost library package: passed for both packages
• Linux plain-CMake/CTest: 6/6 passed
• ost ci validate: 3 cells valid
• git diff --check main...HEAD: clean

Scope

This establishes package and delivery boundaries only. The rigid-body API and Jolt solver remain intentionally unavailable for Phase 1 and Phase 2.