refactor(cli): drop runtime.ts step 3 — cache fallback was unreachable

resolveRuntime's runtimeVersion parameter and cache-fallback step were
dead code from production's perspective. The --runtime-version path is
handled entirely in run.ts (findCachedRelease + downloadRuntimeVersion +
findBundleRoot + validateBundle), and run.ts only ever called
resolveRuntime() with no argument. The parameter and step 3 only existed
because tests exercised them.

Two parallel "compute cache dir" implementations (runtime.ts:65 and
run.ts:157) were a drift hazard if the cache layout ever changes.

Changes:
- Drop the runtimeVersion parameter from resolveRuntime.
- Drop the cache step from runtime.ts entirely; remove the unused
  CACHE_DIR + findBundleRoot imports.
- Narrow RuntimeSource from "env" | "npm" | "cache" to "env" | "npm",
  and drop the now-unused tag field from ResolvedRuntime.
- Remove the three "cache fallback" tests from runtime.test.ts. The
  --runtime-version cache lookup is still covered by run.test.ts via
  findCachedRelease tests.

Tests: 134 → 131 (three deleted tests are obsolete, the production
behavior they were verifying is now covered by run.test.ts).

apps/cli/src/runtime.test.ts

@@ -154,34 +154,6 @@ describe("resolveRuntime", () => {
     });
   });
 
-  describe("cache fallback (step 3 — runtimeVersion only)", () => {
-    it("resolves from cache when runtimeVersion is given and cache exists", () => {
-      const tag = "v0.16.0";
-      const cacheDir = path.join(tmpDir, tag, PLATFORM, "kandev");
-      createFakeBundle(cacheDir);
-
-      const result = resolveRuntime(tag);
-      expect(result.source).toBe("cache");
-      expect(result.tag).toBe(tag);
-      expect(result.bundleDir).toContain(tag);
-    });
-
-    it("throws when runtimeVersion is given but cache dir is missing", () => {
-      expect(() => resolveRuntime("v9.9.9")).toThrow(/Backend binary not found/);
-    });
-
-    it("does not check cache when runtimeVersion is not given", () => {
-      // Create a cache entry — should not be picked up without runtimeVersion
-      const tag = "v0.16.0";
-      const cacheDir = path.join(tmpDir, tag, PLATFORM, "kandev");
-      createFakeBundle(cacheDir);
-
-      // No KANDEV_BUNDLE_DIR, no npm package (require.resolve will fail in test)
-      // Should throw the "no runtime found" error, not resolve from cache
-      expect(() => resolveRuntime()).toThrow(/No Kandev runtime found/);
-    });
-  });
-
   describe("no runtime found", () => {
     it("throws an actionable error message mentioning install paths", () => {
       let error: Error | null = null;

apps/cli/src/runtime.ts

@@ -1,12 +1,9 @@
 import path from "node:path";
 import fs from "node:fs";
 
-import { findBundleRoot, resolveWebServerPath } from "./bundle";
-import { CACHE_DIR } from "./constants";
+import { resolveWebServerPath } from "./bundle";
 import { getBinaryName, getPlatformDir, type PlatformDir } from "./platform";
 
-// Maps platform.ts dir strings to npm @kdlbs/runtime-* package names.
-// platform.ts uses internal names (macos, windows); npm uses os identifiers (darwin, win32).
 const PLATFORM_TO_NPM_PACKAGE: Record<PlatformDir, string> = {
   "linux-x64": "@kdlbs/runtime-linux-x64",
   "linux-arm64": "@kdlbs/runtime-linux-arm64",
@@ -15,28 +12,29 @@ const PLATFORM_TO_NPM_PACKAGE: Record<PlatformDir, string> = {
   "windows-x64": "@kdlbs/runtime-win32-x64",
 };
 
-export type RuntimeSource = "env" | "npm" | "cache";
+export type RuntimeSource = "env" | "npm";
 
 export type ResolvedRuntime = {
   bundleDir: string;
   source: RuntimeSource;
-  tag?: string;
 };
 
 /**
- * Resolve the runtime bundle directory using a three-step priority chain:
+ * Resolve the runtime bundle directory using a two-step priority chain:
  *
- * 1. KANDEV_BUNDLE_DIR env var — set by the Homebrew formula wrapper and useful
- *    for local testing. Skips all other resolution.
- * 2. Installed npm runtime package — looks for @kdlbs/runtime-{platform} in
- *    node_modules via Node module resolution. Only works after `npx kandev@latest`
- *    or `npm install -g kandev` (npm 7+).
- * 3. Cached download — only checked when runtimeVersion is explicitly specified
- *    via --runtime-version. Used by the explicit version download path in run.ts.
+ * 1. KANDEV_BUNDLE_DIR env var — set by the Homebrew formula wrapper and
+ *    useful for local testing. Skips all other resolution.
+ * 2. Installed npm runtime package — looks for @kdlbs/runtime-{platform}
+ *    in node_modules via Node module resolution. Works after
+ *    `npx kandev@latest` or `npm install -g kandev` (requires npm 7+).
+ *
+ * The explicit `--runtime-version <tag>` download path is handled directly
+ * in run.ts (which manages the GitHub download + cache itself); it does
+ * not flow through this function.
  *
  * Throws with an actionable error message if no runtime is found.
  */
-export function resolveRuntime(runtimeVersion?: string): ResolvedRuntime {
+export function resolveRuntime(): ResolvedRuntime {
   const envBundleDir = process.env.KANDEV_BUNDLE_DIR;
   if (envBundleDir) {
     validateBundle(envBundleDir);
@@ -50,7 +48,7 @@ export function resolveRuntime(runtimeVersion?: string): ResolvedRuntime {
     pkgJsonPath = require.resolve(`${packageName}/package.json`);
   } catch {
     // MODULE_NOT_FOUND — npm runtime package is not installed. Fall through
-    // to step 3 (cache fallback only if --runtime-version is set).
+    // to the actionable error below.
   }
   if (pkgJsonPath) {
     // The package IS installed. If validateBundle throws here, the bundle is
@@ -61,13 +59,6 @@ export function resolveRuntime(runtimeVersion?: string): ResolvedRuntime {
     return { bundleDir: packageRoot, source: "npm" };
   }
 
-  if (runtimeVersion) {
-    const cacheDir = path.join(CACHE_DIR, runtimeVersion, platformDir);
-    const bundleDir = findBundleRoot(cacheDir);
-    validateBundle(bundleDir);
-    return { bundleDir, source: "cache", tag: runtimeVersion };
-  }
-
   throw new Error(
     `No Kandev runtime found for ${platformDir}.\n` +
       `  Install via npm (requires npm 7+): npx kandev@latest\n` +