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 --spec-version extension is no longer valid; extension scenarios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/runner/client.ts

@@ -1,7 +1,7 @@
 import { spawn } from 'child_process';
 import { promises as fs } from 'fs';
 import path from 'path';
-import { ConformanceCheck } from '../types';
+import { ConformanceCheck, SpecVersion } from '../types';
 import { getScenario } from '../scenarios';
 import { createResultDir, formatPrettyChecks } from './utils';
 
@@ -17,7 +17,8 @@ async function executeClient(
   scenarioName: string,
   serverUrl: string,
   timeout: number = 30000,
-  context?: Record<string, unknown>
+  context?: Record<string, unknown>,
+  specVersion?: SpecVersion
 ): Promise<ClientExecutionResult> {
   const commandParts = command.split(' ');
   const executable = commandParts[0];
@@ -34,6 +35,9 @@ async function executeClient(
   // 3. Semantic separation: scenario identifies "which test", context provides "test data"
   const env = { ...process.env };
   env.MCP_CONFORMANCE_SCENARIO = scenarioName;
+  if (specVersion) {
+    env.MCP_CONFORMANCE_PROTOCOL_VERSION = specVersion;
+  }
   if (context) {
     // Include scenario name in context for discriminated union parsing
     env.MCP_CONFORMANCE_CONTEXT = JSON.stringify({
@@ -92,7 +96,8 @@ export async function runConformanceTest(
   clientCommand: string,
   scenarioName: string,
   timeout: number = 30000,
-  outputDir?: string
+  outputDir?: string,
+  specVersion?: SpecVersion
 ): Promise<{
   checks: ConformanceCheck[];
   clientOutput: ClientExecutionResult;
@@ -123,7 +128,8 @@ export async function runConformanceTest(
       scenarioName,
       urls.serverUrl,
       timeout,
-      urls.context
+      urls.context,
+      specVersion
     );
 
     // Print stdout/stderr if client exited with nonzero code