Refuse a 0.x minor mismatch, and republish the overview from the named release's tree

src/plugin-host/plugin.test.ts

@@ -86,6 +86,9 @@ test("checkApiVersion: semver compat — equal/patch ok, older minor warns, newe
   assert.equal(checkApiVersion("1.3.0", "1.2.0").level, "refuse"); // needs features a newer host has
   assert.equal(checkApiVersion("2.0.0", "1.5.0").level, "refuse"); // incompatible major (newer)
   assert.equal(checkApiVersion("1.0.0", "2.0.0").level, "refuse"); // incompatible major (older)
+  assert.equal(checkApiVersion("0.1.0", "0.1.9").level, "ok"); // pre-1.0 patch is still ignored
+  assert.equal(checkApiVersion("0.1.0", "0.2.0").level, "refuse"); // pre-1.0 the minor IS the breaking slot
+  assert.equal(checkApiVersion("0.2.0", "0.1.0").level, "refuse");
   for (const bad of ["1", "1.2", "v1.2.3", "01.2.3", "1.2.x", "", 1, undefined, null]) {
     assert.equal(checkApiVersion(bad).level, "refuse", `${String(bad)} must refuse`);
   }

src/plugin-host/plugin.ts

@@ -156,6 +156,12 @@ export function checkApiVersion(pluginVersion: unknown, hostVersion: string = HO
   if (plugin.major !== host.major) {
     return { level: "refuse", message: `plugin targets apiVersion ${pluginVersion}; host is ${hostVersion} — incompatible major` };
   }
+  // Pre-1.0 the major is pinned at 0 until the 1.0.0 milestone, so a breaking change can only land
+  // as a minor (release-tooling/next-version.ts shifts every level down). Treating that as additive
+  // would let a stale plugin boot and fail at runtime instead of at discovery.
+  if (host.major === 0 && plugin.minor !== host.minor) {
+    return { level: "refuse", message: `plugin targets apiVersion ${pluginVersion}; host is ${hostVersion} — pre-1.0 a minor is a contract break, rebuild against ${hostVersion}` };
+  }
   if (plugin.minor > host.minor) {
     return { level: "refuse", message: `plugin targets apiVersion ${pluginVersion} but host is ${hostVersion}; upgrade the host` };
   }