docs: consolidate roadmap and archive obsolete plans

[ktjn]

Summary

• replace the accumulated programme/slice roadmap with one authoritative forward-looking roadmap
• keep only three active priorities: OCI semantic-package distribution, release-artifact preflight parity, and optional package attestation/verification policy
• move completed stabilization work into a concise shipped baseline and continuous engineering invariants
• explicitly mark browser arbitrary-extension execution, subprocess extension hosting, hosted runtime/registry work, and speculative grammar/target breadth as not planned
• consolidate viable research candidates into the roadmap instead of maintaining a separate future-direction document
• archive the stale semantic-platform next-phase plan, Playground UI uplift plan, and future-direction research
• remove competing numbered delivery phases from current compiler, CLI, language, and integration references
• update architecture/playground boundaries to match the supported native-WASM extension model
• correct modelable capabilities: composite-key language semantics are implemented; unsupported targets reject them through target capability negotiation
• update capability tests and changelog accordingly

Added roadmap gaps

• OCI digest/provenance/offline-cache acceptance criteria
• package attestation verification policy
• release artifact preflight parity so PR CI packages the same wheel/sdist, VSIX, and browser artifacts as release CI

The release-preflight item is based on the v1.15.0 packaging failure where the VS Code extension constraint mismatch was only caught by the release workflow.

Archival policy

Historical design and implementation material is preserved under archive paths and explicitly marked non-authoritative. ROADMAP.md is the sole source of future product sequencing.

Verification

• reviewed current implementation/docs against main at 7da3be0
• verified active docs/superpowers/plans/ contains only archived/
• removed non-historical live references to old roadmap Slice/Programme anchors
• retained historical compatibility references only inside archived material
• CI should validate the capability-manifest/test changes