fontconvert.py: cap group uncompressed size at 64 KB

Script-range boundaries alone don't bound group size — dense Unicode
blocks (CJK, user fonts) can produce groups of hundreds of KB. Add a
64 KB hard cap: when adding the next glyph would exceed it, close the
current group and start a new one with the same script ID.

64 KB is well above the largest current built-in group (~50 KB for
notosans_18 Cyrillic) and a safe transient malloc on the ESP32-C3.
The decompressor side already handles any number of groups per font.

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

lib/EpdFont/scripts/fontconvert.py

@@ -726,6 +726,12 @@ if compress:
     # are grouped together for efficient LRU caching on the embedded target.
     # Since glyphs are in codepoint order, glyphs in the same Unicode block
     # are contiguous in the array and form natural groups.
+    #
+    # A hard size cap (GROUP_MAX_UNCOMPRESSED_BYTES) is applied on top of script
+    # boundaries: if adding the next glyph would push the uncompressed group size
+    # over the cap, the group is closed and a new one started with the same script
+    # ID. This keeps the embedded decompressor's transient malloc bounded regardless
+    # of font density (CJK, Vietnamese, user-supplied fonts with large Unicode blocks).
     SCRIPT_GROUP_RANGES = [
         (0x0000, 0x007F),   # ASCII
         (0x0080, 0x00FF),   # Latin-1 Supplement
@@ -743,6 +749,10 @@ if compress:
         (0xFFFD, 0xFFFD),   # Replacement Character
     ]
 
+    # 64 KB cap: large enough to hold any single built-in font group with headroom,
+    # small enough to be a comfortable transient malloc on the ESP32-C3.
+    GROUP_MAX_UNCOMPRESSED_BYTES = 65536
+
     def get_script_group(code_point):
         for i, (start, end) in enumerate(SCRIPT_GROUP_RANGES):
             if start <= code_point <= end:
@@ -753,17 +763,23 @@ if compress:
     current_group_id = None
     group_start = 0
     group_count = 0
+    group_uncompressed = 0
 
     for i, (props, packed) in enumerate(all_glyphs):
         sg = get_script_group(props.code_point)
-        if sg != current_group_id:
+        glyph_aligned_size = ((props.width + 3) // 4) * props.height if props.width > 0 and props.height > 0 else 0
+        size_overflow = group_uncompressed + glyph_aligned_size > GROUP_MAX_UNCOMPRESSED_BYTES
+
+        if sg != current_group_id or size_overflow:
             if group_count > 0:
                 groups.append((group_start, group_count))
             current_group_id = sg
             group_start = i
             group_count = 1
+            group_uncompressed = glyph_aligned_size
         else:
             group_count += 1
+            group_uncompressed += glyph_aligned_size
 
     if group_count > 0:
         groups.append((group_start, group_count))