feat: minted keys, VRF seeds, RFC 9162 proofs, history roots, tag URIs and a transparency log

Keys (spec/02 §2.4), the rule paperstack settled on:
- A work key is a Scholar-style citation key, a hyphen and the first four
  Crockford base32 characters of SHA-256("scienting-key/1:" + identity),
  where the identity is the first of doi, arxiv, isbn, pmid, pmcid and
  openalex in same_as, else the folded title, year and family name. A
  collision lengthens the later key by two characters; a key is minted
  once and never recomputed. A document key adds .si, .v<seq> or .<type>
  for versions other than the version of record.
- Every fixture key is now minted (hale2026harmonic-damb,
  okafor2025solvent-acth, writer2027fixed-ynr7.v3, tiny2026tiny-j4qm), so
  every node and record ID was re-minted and the docs rewritten to match.
  The validator checks the document key against the work key and version
  (doc.key.work) and the hash part against the identifier (doc.key.suffix).
  vectors/keys.jsonl pins the cases paperstack's tests pin.

Review:
- Spot-check seeds are ECVRF-EDWARDS25519-SHA512-TAI proofs (RFC 9381),
  implemented in tools/oracle/vrf.py in plain Python and reproduced
  against the RFC's Example 16; the sample ranks by the VRF output. The
  earlier Ed25519 seed could be ground by a dishonest checker.
- RFC 9162 inclusion and consistency proofs (crypto.inclusion_path,
  verify_inclusion, consistency_proof, verify_consistency) with
  scienting-proof/1, vectors/proofs.jsonl and two fixture records.
- document.json carries `history`, the root over the corrections, with
  the empty-tree hash for none (spec/11 §11.5, rule doc.history).
- A provisional transparency log (spec/23): log entries, signed-note
  checkpoints with key hashes as transparency logs share them, the rule
  that a logged revocation beats a self-asserted time, and vectors.
- Reserved signature schemes cose and ml-dsa-65 (spec/08 §8.4).

Identifiers and text:
- urn:scienting: becomes tag:scient.ing,2026: (RFC 4151), since the URN
  namespace was never registered.
- Unicode 16.0 is pinned for normalization and categories (spec/01 §1.4);
  the checker refuses another version.

Checking:
- check.py: keys, proofs, VRF and checkpoint vectors; the proof and
  checkpoint records; a check that every ID and hash the prose quotes
  exists in the fixtures or vectors; three more broken packages
  (history-wrong, key-suffix, key-work).
- stress.py: key, VRF, proof and checkpoint properties.
- Docs: a keys section in the concepts and FAQ, rationale entries for
  keys, VRF, proofs, tag URIs and the Unicode pin, a "Looking ahead"
  section, media-type registration and OCI distribution notes.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

schemas/checkpoint.schema.json

@@ -0,0 +1,62 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://paperspec.scient.ing/schemas/checkpoint.schema.json",
+  "title": "Log checkpoint",
+  "description": "scienting-checkpoint/1: a signed checkpoint of a team's transparency log, as the parsed form of its signed note; spec/23-log.md.",
+  "type": "object",
+  "properties": {
+    "format": {
+      "const": "scienting-checkpoint/1"
+    },
+    "origin": {
+      "type": "string",
+      "pattern": "^[^\\n]+$",
+      "minLength": 1
+    },
+    "size": {
+      "type": "integer",
+      "minimum": 0
+    },
+    "root": {
+      "$ref": "common.schema.json#/$defs/hash"
+    },
+    "signatures": {
+      "type": "array",
+      "items": {
+        "type": "object",
+        "properties": {
+          "name": {
+            "$ref": "common.schema.json#/$defs/key"
+          },
+          "key_hash": {
+            "type": "string",
+            "pattern": "^[0-9a-f]{8}$"
+          },
+          "value": {
+            "$ref": "common.schema.json#/$defs/base64"
+          }
+        },
+        "required": [
+          "name",
+          "key_hash",
+          "value"
+        ],
+        "additionalProperties": false
+      },
+      "minItems": 1
+    },
+    "note": {
+      "type": "string",
+      "minLength": 1
+    }
+  },
+  "required": [
+    "format",
+    "origin",
+    "size",
+    "root",
+    "signatures",
+    "note"
+  ],
+  "additionalProperties": false
+}