Charset lower: ASCII map only A–Z/a–z; CHR$ uses PETSCII path

gfx_ascii_to_screencode_lowcharset for all 32–126 broke CHR (!) and
shifted CHR; non-letters must use petscii_to_screencode.

Co-authored-by: Chris Garrett <chris@chrisg.com>

Author: cursoragent

CHANGELOG.md

@@ -5,7 +5,7 @@
 - **Browser canvas**: `#OPTION charset lower` (and charset from CLI before run) now applies when video state attaches — fixes PETSCII space (screen code 32) drawing as wrong glyph (e.g. `!`) because the uppercase char ROM was still active.
 - **INPUT (canvas / GFX)**: When **Stop** / watchdog sets `wasmStopRequested` while waiting in `gfx_read_line`, report **"Stopped"** instead of **"Unexpected end of input"** (misleading; not EOF).
 
-- **PETSCII + lowercase charset**: With lowercase video charset (`#OPTION charset lower` / `-charset lower`), `PRINT` / `CHR$` map **ASCII** bytes via `gfx_ascii_to_screencode_lowcharset()` whenever `petscii_to_screencode()` would be wrong — including when the host **omits `-petscii`** (previously used upper-ROM ASCII mapping with the lower char ROM). `wasm_browser_canvas_test` covers charset lower with and without the PETSCII checkbox.
+- **PETSCII + lowercase charset**: With lowercase video charset, **ASCII letters** in string literals use `gfx_ascii_to_screencode_lowcharset()`; **all other bytes** (including **`CHR$(n)`** and punctuation) use **`petscii_to_screencode()`** so `CHR$(32)` is space and `CHR$(65)` matches PETSCII semantics (was wrongly treated as ASCII, shifting output). Works with or without **`-petscii`** on the host. `wasm_browser_canvas_test` covers charset lower with and without the PETSCII checkbox.
 
 - **Project**: Renamed to **RGC-BASIC** (Retro Game Coders BASIC). GitHub: **`omiq/rgc-basic`**. Tagline: modern cross-platform BASIC interpreter with classic syntax compatibility, written in C. WASM CI artifact: **`rgc-basic-wasm.tar.gz`**. Tutorial CSS classes: `rgc-tutorial-*` (`cbm-tutorial-*` removed). JS: prefer **`RgcBasicTutorialEmbed`** / **`RgcVfsHelpers`** (deprecated **`Cbm*`** aliases retained briefly).
 

basic.c

@@ -1602,8 +1602,13 @@ static void gfx_put_byte(unsigned char b)
     } else if (gfx_raw_screen_codes) {
         sc = petscii_to_screencode(b);
     } else if (gfx_vs && gfx_vs->charset_lowercase && b >= 32 && b <= 126) {
-        /* Lowercase char ROM: map ASCII literals for any -petscii setting (IDE may omit -petscii). */
-        sc = gfx_ascii_to_screencode_lowcharset(b);
+        /* Lowercase char ROM + ASCII string literals: only A–Z / a–z are ASCII letters.
+         * CHR$(n) and punctuation use PETSCII semantics (see FN_CHR raw byte in gfx). */
+        if ((b >= 'A' && b <= 'Z') || (b >= 'a' && b <= 'z')) {
+            sc = gfx_ascii_to_screencode_lowcharset(b);
+        } else {
+            sc = petscii_to_screencode(b);
+        }
     } else if (petscii_mode) {
         sc = petscii_to_screencode(b);
     } else if (b >= 32 && b <= 126) {