lua: byte-to-char only single chars (fix \0-escape regression breaking string lengths)

plans/lua-on-sx.md

@@ -82,6 +82,7 @@ Each item: implement → tests → tick box → update progress log.
 
 _Newest first. Agent appends on every commit._
 
+- 2026-04-24: lua: scoreboard iteration — `lua-byte-to-char` regression fix. My previous change returned 2-char strings (`"\a"` etc.) for bytes that SX string literals can't express (0, 7, 8, 11, 12, 14–31, 127+), breaking `'a\0a'` length from 3 → 4. Now only 9/10/13 and printable 32-126 produce real bytes; others use a single `"?"` placeholder so `string.len` stays correct. literals.lua back to failing at assert #4 (was regressed to #2).
 - 2026-04-24: lua: scoreboard iteration — **decimal string escapes** `\ddd` (1-3 digits). Tokenizer `read-string` previously fell through to literal for digits, so `"\65"` came out as `"65"` not `"A"`. Added `read-decimal-escape!` consuming up to 3 digits while keeping value ≤255, plus `\a`/`\b`/`\f`/`\v` control escapes and `lua-byte-to-char` ASCII lookup. 362 tests (+2 escape tests).
 - 2026-04-24: lua: scoreboard iteration — **`loadstring` error propagation**. When `loadstring(s)()` was implemented as `eval-expr ( (let () compiled))`, SX's `eval-expr` wrapped any propagated `raise` as "Unhandled exception: X" — so `error('hi')` inside a loadstring'd chunk came out as that wrapped string instead of the clean `"hi"` Lua expects. Fix: transpile source once into a lambda AST, `eval-expr` it ONCE to get a callable fn value, return that — subsequent calls propagate raises cleanly. Guarded parse-failure path returns `(nil, err)` per Lua convention. vararg.lua now runs past assert #18; errors.lua past parse stage.
 - 2026-04-24: lua: scoreboard iteration — `table.sort` O(n²) insertion-sort → **quicksort** (Lomuto partition). 1000-element sorts finish in ms; but `sort.lua` uses 30k elements and still times out even at 90s (metamethod-heavy interpreter overhead). Correctness verified on 1000/5000 element random arrays.