test(e2e): add full smoketest covering install, search, upgrade, remove

Creates tests/e2e/smoketest.sh — a hermetic end-to-end test that:
- Sets up a mock ZUUR (python3 http.server serving local index/recipes)
- Creates fake zeta-makepkg and zeta scripts (no real Lua tools needed)
- Tests the full pipeline: search → install → upgrade → info → remove
- Verifies exit codes (0, 2 for not-found)
- Verifies installed.json state tracking
- Verifies binary installation/removal via ZETA_ROOT

BUG FOUND (documented in problems.md, not fixed):
- info.d scanDepsArray stores ptrdiff_t indexOf() result in size_t,
  causing ArrayIndexError when recipe lacks a deps field.
  Workaround: recipe includes deps = {}.

Evidence: .omo/evidence/task-27-tofu-core.log — 16/16 checks pass.

.omo/evidence/task-27-tofu-core.log

@@ -0,0 +1,44 @@
+=== tofu smoketest ===
+
+step 0: build tofu binary... PASS
+step 1: create temp workspace and hello binary... PASS
+step 2: create hello.recipe... PASS
+step 3: create package.lua... PASS
+step 4: create fake zeta-makepkg and zeta... PASS
+step 5: create mock ZUUR layout... PASS
+step 6: start mock ZUUR http server... PASS
+      ZUUR_URL=http://127.0.0.1:27644
+      CACHE_DIR=/tmp/tmp.707gC0rrPK/cache
+
+step 7: tofu -Ss hello (search)... PASS
+step 8: tofu -S hello --noconfirm (install)...   .  fetching index http://127.0.0.1:27644/index.lua
+ ok  index loaded: 1 packages
+==> fetching recipe hello
+  .  downloading hello.recipe
+  .  downloading package.lua
+  .  downloading build.sh
+  -  no build.sh for hello (optional build script)
+ ok  recipe hello downloaded
+==> generating build plan
+  -    + hello (recipe)
+ ok  build plan: 1 packages
+  -  will build 1 package(s): hello
+==> ──── building hello (1/1) ────
+ ok  hello
+installing hello-1.0 ok  installed hello with 0 dependencies
+ ok  installed 1 package(s)
+        installed.json check... PASS
+        binary in root check... PASS
+step 9: tofu -Syu --noconfirm (upgrade — nothing to do)... PASS
+step 10: tofu -Si hello (info)... PASS
+step 11: tofu -R hello --noconfirm (remove)... removing hello ok  removed hello
+        binary removed check... PASS
+        installed.json cleared check... PASS
+step 12: tofu -S nonexistent --noconfirm (expect exit 2)...   .  fetching index http://127.0.0.1:27644/index.lua
+ ok  index loaded: 1 packages
+PASS
+step 13: cleanup... PASS
+
+=== smoketest complete ===
+PASS: 16 checks passed
+All checks passed.