plans: tick Phase 6 prelude + progress log (635/635)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/haskell-on-sx.md

@@ -107,13 +107,21 @@ Key mappings:
 ### Phase 6 — real IO + Prelude completion
 - [x] Real `IO` monad backed by `perform`/`resume`
 - [x] `putStrLn`, `getLine`, `readFile`, `writeFile`, `print`
-- [ ] Full-ish Prelude: `Maybe`, `Either`, `List` functions, `Map`-lite
+- [x] Full-ish Prelude: `Maybe`, `Either`, `List` functions, `Map`-lite
 - [ ] Drive scoreboard toward 150+ passing
 
 ## Progress log
 
 _Newest first._
 
+- **2026-05-06** — Phase 6 prelude extras (635/635). `nub`, `sort`, `sortBy`, `sortOn`,
+  `splitAt`, `span`, `break`, `partition`, `unzip`, `tails`, `inits`, `isPrefixOf`,
+  `isSuffixOf`, `isInfixOf`, `intercalate`, `intersperse`, `unwords`, `unlines`,
+  `interactApply/interact`. SX builtins: `ord`, `isAlpha`, `isAlphaNum`, `isDigit`,
+  `isSpace`, `isUpper`, `isLower`, `digitToInt`, `words`, `lines`. Fixed `++` on SX
+  strings (`hk-list-append` now handles string concat via `str`). Unified list repr:
+  `--sx-to-hk--` now uses `":"/"[]"` matching `hk-mk-cons`. 47 new tests.
+
 - **2026-05-06** — Phase 6 `getLine`/`getContents`/`readFile`/`writeFile`. `hk-force`
   extended: 0-arity builtins (`arity=0` dicts) are called immediately when forced,
   making `getLine`/`getContents` work naturally as IO actions (no arity-0 application