docs: make component composition evidence-gated

[pcvantol]

Summary

Defines how Forge Platform participates in cross-repository Forge Missions without becoming the planner.

• Producer and installer-role implementation may proceed in parallel when Forge declares no hard dependency.
• Final component-manifest Actions are evidence-gated by actual published producer artifacts.
• Manifest entries distinguish source revision from the digest of the installable artifact bytes and carry qualification/provenance references.
• Source merge or guessed future checksum cannot finalize a manifest.
• Applies the rule to Forge Runtime, Workspace Server, Workspace Client, EP Server and EP Project Agent.
• Updates the component-manifest structural schema with source_revision and required artifact qualification evidence.
• Adds the evidence-gated composition seam to the architecture and roadmap index.

Reference cross-repo Mission

For EP Execution Agents, Forge may run EP implementation/package work and Forge Platform Agent installer-role work independently; the final Forge Platform component manifest remains hard-dependent on publication of qualified EP Server/Agent artifacts.

Boundary

Forge owns the Engineering Action graph; EP owns execution/admission/evidence; Forge Platform owns release composition. This PR changes contracts/docs/schema only and does not claim installer implementation.