runtime: build_runtime_graph labels stale node data with the current files' source_revision #59

Problem

build_runtime_graph(graph, ...) copies node content from an already parsed Graph, but rereads article bytes from disk to compute source_revision.

If an article changes between load_graph and projection, the result pairs stale node data with the new bytes' revision. A fresh load then produces the same source_revision with different node content, so the revision no longer identifies the snapshot it labels.

The parsed Node already carries source_sha256; projection never compares it with the reread bytes. This predates #15 and affects direct or cached Graph consumers even though in-repository callers usually load and project back to back.

Acceptance criteria

• Reject projection when reread article bytes do not match node.source_sha256.
• Define the contract for hand-built nodes without a source digest.
• Add a regression that edits an article between load_graph and build_runtime_graph and proves no runtime graph is emitted.

PR #28 improves Lean-source snapshots but does not currently add this roadmap-node digest check.