lua: string library (len/upper/lower/rep/sub/byte/char/find/match/gmatch/gsub/format) +19 tests

plans/lua-on-sx.md

@@ -68,7 +68,7 @@ Each item: implement → tests → tick box → update progress log.
 - [x] `coroutine.create`/`.resume`/`.yield`/`.status`/`.wrap` via `perform`/`cek-resume`
 
 ### Phase 6 — standard library
-- [ ] `string` — `format`, `sub`, `find`, `match`, `gmatch`, `gsub`, `len`, `rep`, `upper`, `lower`, `byte`, `char`
+- [x] `string` — `format`, `sub`, `find`, `match`, `gmatch`, `gsub`, `len`, `rep`, `upper`, `lower`, `byte`, `char`
 - [ ] `math` — full surface
 - [ ] `table` — `insert`, `remove`, `concat`, `sort`, `unpack`
 - [ ] `io` — minimal stub (read/write to SX IO surface)
@@ -82,6 +82,7 @@ Each item: implement → tests → tick box → update progress log.
 
 _Newest first. Agent appends on every commit._
 
+- 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.
 - 2026-04-24: lua: phase 5 — coroutines (create/resume/yield/status/wrap) via `call/cc` (perform/cek-resume not exposed to SX userland). Handles multi-yield + final return + arg passthrough. Fix: body's final return must jump via `caller-k` to the **current** resume's caller, not unwind through the stale first-call continuation. 273 tests.
 - 2026-04-24: lua: generic `for … in …` — parser split (`=` → num, else `in`), new `lua-for-in` node, transpile to `let`-bound `f,s,var` + recursive `__for_loop`. Added `ipairs`/`pairs`/`next`/`lua-arg` globals. Lua fns now arity-tolerant (`&rest __args` + indexed bind) — needed because generic for always calls iter with 2 args. Noted early-return-in-nested-block as pre-existing limitation. 265 tests.
 - 2026-04-24: lua: `pcall`/`xpcall`/`error` via SX `guard` + `raise`. Added `lua-apply` (arity-dispatch 0-8, apply fallback) because SX `apply` re-wraps raises as "Unhandled exception". Table payloads preserved (`error({code = 42})`). 256 total tests.
@@ -105,4 +106,7 @@ _Shared-file issues that need someone else to fix. Minimal repro only._
 
 ## Known limitations (own code, not shared)
 
+- **`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.
 - **Early `return` inside nested block** — `if cond then return nil end ...rest` doesn't exit the enclosing function; `rest` runs anyway. Use `if cond then return X else return Y end` instead. Likely needs guard+raise sentinel for proper fix.