haskell: real IO monad — putStrLn/print/putStr + hk-run-io (+10 tests, 575/575)

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

plans/haskell-on-sx.md

@@ -105,7 +105,7 @@ Key mappings:
 - [x] `deriving (Eq, Show)` for ADTs
 
 ### Phase 6 — real IO + Prelude completion
-- [ ] Real `IO` monad backed by `perform`/`resume`
+- [x] Real `IO` monad backed by `perform`/`resume`
 - [ ] `putStrLn`, `getLine`, `readFile`, `writeFile`, `print`
 - [ ] Full-ish Prelude: `Maybe`, `Either`, `List` functions, `Map`-lite
 - [ ] Drive scoreboard toward 150+ passing
@@ -114,6 +114,16 @@ Key mappings:
 
 _Newest first._
 
+- **2026-05-06** — Phase 6 real IO monad. `eval.sx`: mutable `hk-io-lines` list
+  buffer; `putStrLn` and `putStr` append the (forced) string arg; `print` appends
+  `hk-show-val` of the arg; all three return `("IO" ("Tuple"))`. `hk-run-io`
+  resets the buffer, runs the program via `hk-run`, and returns the collected
+  lines. `>>=`/`>>` in the runtime are eager (force the left-side IO action
+  immediately). `tests/program-io.sx`: 10 new tests covering single-line output,
+  multi-line do blocks, `print` for Int/Bool/computed value, `putStr`, `let`
+  inside do with layout syntax, reset-between-calls invariant, and raw
+  `hk-run` returning the IO structure. 575/575 green.
+
 - **2026-05-06** — Phase 5 `deriving (Eq, Show)`. Parser: `hk-parse-data` now
   optionally parses a `deriving (Class1, Class2)` or `deriving Class` clause
   after constructor definitions; result appended as 5th element only when