Phase 5: HAMT Persistent Vector + CI fixes

- Add lib/cljnim_pvec.nim: 32-way HAMT Persistent Vector with structural sharing
- Migrate ckVector from seq[CljVal] to PersistentVector[CljVal] in runtime
- Fix recursive pushLeaf bug (nil nodes at depth > 2)
- Fix defn/if return value bug: replace discard with result = in proc bodies
- Fix cljNth to accept CljVal index
- Add len and [] overloads for PersistentVector seq compatibility
- Add tests/test_pvec.nim (14 tests)
- Update .gitlab-ci.yml to nim:2.2.10 and add test_pvec
- Update docs/ROADMAP.md and add PHASE5_HAMT.md

docs/ROADMAP.md

@@ -45,11 +45,11 @@
 - [ ] Tool-call format for AI framework integration
 
 ## Phase 5: Persistent Data Structures
-- [ ] Persistent Vector (Hash Array Mapped Trie)
+- [x] Persistent Vector (Hash Array Mapped Trie, 32-way branching)
 - [ ] Persistent Map (HAMT)
 - [ ] Persistent Set
-- [ ] `conj`, `assoc`, `dissoc`, `get`, `get-in`
-- [ ] `nth`, `first`, `rest`, `last`, `count` on persistent collections
+- [x] `conj`, `assoc`, `dissoc`, `get`, `get-in`
+- [x] `nth`, `first`, `rest`, `last`, `count` on persistent collections
 - [ ] Transients for batch mutations
 
 ## Phase 6: Clojure Core Library