Check protocol version before content digest and reject mod-pack maps

ddiff --git a/apps/server/src/game/platformIntegration.test.ts b/apps/server/src/game/platformIntegration.test.ts
iindex c0dc028..f85ca48 100644
---- a/apps/server/src/game/platformIntegration.test.ts
++++ b/apps/server/src/game/platformIntegration.test.ts
@@@ -221,6 +221,15 @@ describe('Games on platform identity, events and mods (real server, restart)', (
        })
        expect(stale.inbox.some((m) => m.t === 'welcome')).toBe(false)
      }
++    // An older bundle sends neither the current version nor a digest; it must
++    // get protocol_mismatch, the only reason the client reloads itself on.
++    const oldBundle = connect(a.game)
++    oldBundle.conn.identity = { guestKeyHash: hashGuestKey('old-bundle') }
++    a.game.onMessage(oldBundle.conn, {
++      t: 'hello', v: PROTOCOL_VERSION - 1, slot: 0, name: 'Old', appearance: defaultAppearance(),
++    })
++    await new Promise((r) => setTimeout(r, 0))
++    expect(oldBundle.inbox).toContainEqual({ t: 'reject', reason: 'protocol_mismatch' })
  
      // The signed-in player is keyed by the canonical subject.
      const ana = connect(a.game)
ddiff --git a/apps/server/src/game/systems/sessions.ts b/apps/server/src/game/systems/sessions.ts
iindex 5ed8dc8..cb5144f 100644
---- a/apps/server/src/game/systems/sessions.ts
++++ b/apps/server/src/game/systems/sessions.ts
@@@ -87,6 +87,12 @@ export class SessionsSystem implements System {
    private async resolveHello(conn: GameConnection, msg: ClientHello): Promise<void> {
      const ctx = this.ctx
      const serverDigest = ctx.world.content.digest
++    // Protocol first: a stale bundle predates the digest and only reloads itself on protocol_mismatch.
++    if (msg.v !== PROTOCOL_VERSION) {
++      conn.send(encodeServerMessage({ t: 'reject', reason: 'protocol_mismatch' }))
++      conn.close(4002, 'protocol_mismatch')
++      return
++    }
      if (msg.contentDigest !== serverDigest) {
        conn.send(encodeServerMessage({
          t: 'reject', reason: 'content_mismatch',
@@@ -95,11 +101,6 @@ export class SessionsSystem implements System {
        conn.close(4013, 'content_mismatch')
        return
      }
--    if (msg.v !== PROTOCOL_VERSION) {
--      conn.send(encodeServerMessage({ t: 'reject', reason: 'protocol_mismatch' }))
--      conn.close(4002, 'protocol_mismatch')
--      return
--    }
      if (ctx.sessions.size >= ctx.config.maxPlayers) {
        conn.send(encodeServerMessage({ t: 'reject', reason: 'server_full' }))
        conn.close(4003, 'server_full')
ddiff --git a/apps/server/src/mods/contentPack.test.ts b/apps/server/src/mods/contentPack.test.ts
iindex 197e37d..0937b0c 100644
---- a/apps/server/src/mods/contentPack.test.ts
++++ b/apps/server/src/mods/contentPack.test.ts
@@@ -44,7 +44,9 @@ describe('games-content@2 definitions', () => {
    it('recognizes an authored map as a definition-capability section', () => {
      const mapPack = { map: SCRAPLANDIA_PACK.map! }
      expect(capabilitiesUsedBy(mapPack)).toEqual([CAP_DEFINE])
--    expect(validateContentPack(mapPack, createContent()).ok).toBe(true)
++    const result = validateContentPack(mapPack, createContent())
++    expect(result.ok).toBe(false)
++    expect(!result.ok && result.errors).toContain('map is defined by the built-in pack and cannot be replaced by a mod')
      expect(validateContentPack({ map: { id: 'broken' } }, createContent()).ok).toBe(false)
    })
    it('needs games.def.define for any non-empty defs section', () => {
ddiff --git a/apps/server/src/mods/contentPack.ts b/apps/server/src/mods/contentPack.ts
iindex 4c0f7f7..bfe6062 100644
---- a/apps/server/src/mods/contentPack.ts
++++ b/apps/server/src/mods/contentPack.ts
@@@ -154,6 +154,8 @@ export function validateContentPack(
    }
    const pack = value as unknown as ContentPack
    const errors: string[] = []
++  // The built-in pack always supplies the map, so a mod's map would be skipped at boot as a duplicate.
++  if (pack.map) errors.push('map is defined by the built-in pack and cannot be replaced by a mod')
    if (pack.defs || pack.map) {
      // The content the pack would be merged onto; its own ids may not already exist.
      const merged = mergePackDefs(