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

src/emitter.nim

@@ -270,6 +270,9 @@ proc emitSpecialForm(items: seq[CljVal], indent: int): string =
       bodyCode = emitExpr(body[0], indent + 1)
     else:
       bodyCode = emitBlock(body, indent + 1)
+    # In a proc body, replace "discard " with "result = " so that
+    # if/when branches return their last expression properly.
+    bodyCode = bodyCode.replace("discard ", "result = ")
     let procName = mangleName(name.symName)
     return sp & "proc " & procName & "(" & paramNames.join(", ") & "): CljVal =\n" & bodyCode