Evidence: define the three evidence kinds and their formats

[hyperpolymath]

Charter artefact (2) explicit evidence. Sketch area: src/evidence/ (src/evidence/README.adoc).

Define the witness behind every claim. No envelope without evidence.

Scope

• The three kinds: proof (Coq/Agda/Idris2), check (decision-procedure result), runtime discipline (declared, enforced invariant).
• On-disk/in-memory representation of each kind.
• The link from a Core checking result to the evidence it produced.
• Validation that an envelope's cited evidence actually supports its claim.

Done when

• A spec/format for each evidence kind exists.
• The manifest emitter can refuse an envelope whose evidence is missing/insufficient.
• Evidence kind is recorded honestly (a check is never labelled a proof).

Ref: docs/architecture/DESIGN-DISCIPLINE.adoc, docs/status/PROOF-NEEDS.adoc.