lua: io stub (buffered) + print/tostring/tonumber +12 tests

plans/lua-on-sx.md

@@ -71,7 +71,7 @@ Each item: implement → tests → tick box → update progress log.
 - [x] `string` — `format`, `sub`, `find`, `match`, `gmatch`, `gsub`, `len`, `rep`, `upper`, `lower`, `byte`, `char`
 - [x] `math` — full surface
 - [x] `table` — `insert`, `remove`, `concat`, `sort`, `unpack`
-- [ ] `io` — minimal stub (read/write to SX IO surface)
+- [x] `io` — minimal stub (read/write to SX IO surface)
 - [ ] `os` — time/date subset
 
 ### Phase 7 — modules + full conformance
@@ -82,6 +82,7 @@ Each item: implement → tests → tick box → update progress log.
 
 _Newest first. Agent appends on every commit._
 
+- 2026-04-24: lua: `io` stub + `print`/`tostring`/`tonumber` globals. io buffers to internal `__io-buffer` (tests drain it via `io.__buffer()`). print: tab-sep + NL. tostring respects `__tostring` metamethod. 334 tests.
 - 2026-04-24: lua: `table` lib — insert (append / at pos, shifts up), remove (last / at pos, shifts down), concat (sep, i, j), sort (insertion sort, optional cmp), unpack + table.unpack, maxn. Caught trap: local helper named `shift` collides with SX's `shift` special form → renamed to `tbl-shift-up`/`tbl-shift-down`. 322 tests.
 - 2026-04-24: lua: `math` lib — pi/huge + abs/ceil/floor/sqrt/exp/log/log10/pow/trig (sin/cos/tan/asin/acos/atan/atan2)/deg/rad/min/max (&rest)/fmod/modf/random (0/1/2 arg)/randomseed. Most ops delegate to SX primitives; log w/ base via change-of-base. 309 tests.
 - 2026-04-24: lua: `string` lib — len/upper/lower/rep/sub (1-idx + neg)/byte/char/find/match/gmatch/gsub/format. Patterns are literal-only (no `%d`/etc.); format is `%s`/`%d`/`%f`/`%%` only. `string.char` uses printable-ASCII lookup + tab/nl/cr. 292 tests.
@@ -108,6 +109,7 @@ _Shared-file issues that need someone else to fix. Minimal repro only._
 
 ## Known limitations (own code, not shared)
 
+- **`io` library is a stub** — `io.write`/`print` append to an internal `__io-buffer` (accessible via `io.__buffer()` which returns + clears it) instead of real stdout. `io.read`/`open`/`lines` return nil. Suitable for tests that inspect output; no actual stdio.
 - **`string.find`/`match`/`gmatch`/`gsub` patterns are LITERAL only** — no `%d`/`%a`/`.`/`*`/`+`/etc. Implementing Lua patterns is a separate work item; literal search covers the common case.
 - **`string.format`** supports only `%s`, `%d`, `%f`, `%%`. No width/precision flags (`%.2f`, `%5d`).
 - **`string.char`** supports printable ASCII 32–126 plus `\t`/`\n`/`\r`; other codes error.