Phase 3c: formalize the three version axes and the negotiation flow

[MGudgin]

• I have signed the Contributor License Agreement.
• This pull request is related to an issue.
• If this PR changes build commands, project architecture, or key conventions, I have updated .github/copilot-instructions.md.

---

Stacked PR. Base is user/gudge/versioning_phase3b_schema_floor (Phase 3b), not main. Review/merge the stack in order (2.5 -> 3a -> 3b -> 3c); this branch will be retargeted to main as its base lands.

Summary

Phase 3c of the versioning remediation — the documentation half. It rewrites the conceptual sections of docs/versioning.md to match the post-3a/3b reality: it formalizes the three independent, never-conflated version axes and replaces the old version-centric "Version Negotiation" steps with the three-stage, capability-driven runtime flow. Docs-only; no code change.

Details

• Add a "Three version axes" section: schema/config version (the version field; semver; 0.6.x floor through 0.8.x dev ceiling; major.minor compared), product version (Rust workspace + npm; independent; check-version-sync.js), and host capability (negotiated at runtime, never a string in the config). States the Phase 3a invariant: schema version no longer selects the Windows backend.
• Rewrite "Version Negotiation" as three ordered stages: (1) schema-range check at the trust boundary; (2) containment resolve (abstract process/vm intent → concrete backend per host, independent of version); (3) host-capability negotiate (BaseContainer when is_base_container_usable(), else AppContainer BFS/DACL, logged; the only fallback). Updates the Data Flow diagram to match.
• Correct the BaseContainer OS handshake: today MXC uses a fixed SANDBOX_SPEC_VERSION ("0.1.0") and calls Experimental_CreateProcessInSandbox directly; the EnumerateSandboxSpecVersionInfo spec-version handshake is now marked forward-looking / not yet implemented (it had been written as current behavior).
• Tighten the Error Contract: schema-range, capability (BackendUnavailable on the ScriptResponse / SDK spawn paths), and policy-unsupported (e.g. deniedPaths) failures are typed and actionable; security policy never fuzzy-falls-back.
• Fixups surfaced while editing: bump a stale 0.4.0 example to 0.6.0-alpha; note retired 0.4/0.5 stable schemas are kept as artifacts; correct a promotion-step reference to a non-existent parser symbol.

Tests

• Docs-only change; no code or CI gates affected. The draft was fact-checked (rubber-duck pass) against config_parser.rs, models.rs, fallback_detector.rs, dispatcher.rs, and base_container_runner.rs.

Microsoft Reviewers: Open in CodeFlow