Add research execution contracts

[foxsplendid]

What changed

• Add additive experiment-run/1.0 and claim-evidence/1.0 JSON Schemas.
• Add synthetic examples, stdlib validation coverage, and execution/evidence conventions.
• Synchronize product direction, trust boundaries, README, and release-candidate facts for v2.3.0.

Why

Scriptorium needs stable, model-neutral file contracts for externally executed research runs and human-reviewed claim/evidence links. This PR defines those exchange shapes without claiming that the runtime components already consume them.

Product impact

This is an additive contract release candidate. Existing formats remain compatible. Provenance and the umbrella continue to describe runtime ingestion of these two new contracts as future work.

Validation

• 14 worked examples validated.
• 83 tests passed.
• Working-tree release consistency passed.
• Public payload/privacy scan passed.

Merge note

The umbrella candidate pins commit 0439e7abe4dd307c8b97afa34f9fc3b45605d3b4. Preserve this commit SHA with a merge commit or fast-forward; do not squash or rebase this PR.