feat: support "draft" as a first-class spec version target (#255)

* feat: support "draft" as a first-class spec version target

- Add LATEST_SPEC_VERSION and DATED_SPEC_VERSIONS constants so the next
  spec release is a one-line change in types.ts.
- Accept "draft" as a valid protocolVersion in the initialize check and
  mock-server response.
- --spec-version draft now selects latest-dated scenarios plus draft-tagged
  ones, so SEP authors can run the full suite against an SDK tracking the
  in-progress spec without retagging core scenarios.
- Forward --spec-version to the client process via
  MCP_CONFORMANCE_SPEC_VERSION so SDK examples can pick the matching
  protocolVersion.

Closes #253

* review: collapse SpecVersion to wire strings; dedup NEGOTIABLE_PROTOCOL_VERSIONS; split out 'extension'

Addresses review feedback from @felixweinberger and @mikekistler:

- SpecVersion is now always a wire protocolVersion string
  (DatedSpecVersion | typeof DRAFT_PROTOCOL_VERSION). The separate
  'draft' tag literal is gone from the type system; 'draft' survives
  only as a CLI input alias in resolveSpecVersion. Removes the
  tag-vs-wire confusion and the specVersionToProtocolVersion mapping.
- NEGOTIABLE_PROTOCOL_VERSIONS in types.ts is the single source for
  what the mock server accepts on initialize (was duplicated in
  checks/client.ts and scenarios/client/initialize.ts).
- 'extension' moved out of SpecVersion into a separate ScenarioSpecTag
  type. --spec-version extension is no longer valid; extension scenarios
  remain reachable via --suite extensions.
- Draft-tagged scenarios now use the DRAFT_PROTOCOL_VERSION constant
  so a draft revision bump is a one-line change in types.ts.

src/index.ts

@@ -152,7 +152,8 @@ program
                 options.command,
                 scenarioName,
                 timeout,
-                outputDir
+                outputDir,
+                specVersionFilter
               );
               return {
                 scenario: scenarioName,
@@ -259,7 +260,8 @@ program
         validated.command,
         validated.scenario,
         timeout,
-        outputDir
+        outputDir,
+        specVersionFilter
       );
 
       const { overallFailure } = printClientResults(