fix: classify Hangul Compatibility Jamo (U+3130–U+318F) as CJK

[mayrang]

Closes #142

Problem

isCJKCodePoint() in src/analysis.ts includes Hangul syllables (U+AC00–U+D7AF) but not Hangul Compatibility Jamo (U+3130–U+318F) — the standalone consonants and vowels Korean speakers type constantly in digital communication:

Expression	Meaning	Characters
ㅋㅋㅋ	laughter ("lol")	U+314B
ㅠㅠ	crying / sad	U+3160
ㄹㅇ	"literally" / "for real"	U+3139 U+3147
ㅇㅋ	okay	U+3147 U+314B
ㄴㄴ	no / nope	U+3134

These appear in virtually every Korean chat conversation. Without CJK classification, these characters are treated as atomic segments the line-breaker cannot split at grapheme boundaries — causing line counts 1 too high compared to Chrome and Safari.

Reproduction

Wrong line count (most severe)

font: "20px serif", width: 200px
text: "ㅋㅋㅋ 진짜 웃기다 ㅋㅋㅋ 진짜로"

pretext: 4 lines
browser: 3 lines  ← 1 line off

Wrong line count

font: "20px serif", width: 150px
text: "이거 ㄹㅇ임 ㄹㅇ 아니면 뭐야"

pretext: 5 lines
browser: 4 lines  ← 1 line off

Reproduced in: Chrome ✓ Safari ✓

The bug is width-sensitive — all Hangul Compatibility Jamo are affected, but failures appear at different widths depending on the character.

Reproduce without this PR's script (using the existing /probe page on main):

Run bun start, then open:

http://localhost:3210/probe?text=ㅋㅋㅋ%20진짜%20웃기다%20ㅋㅋㅋ%20진짜로&width=200&font=20px%20serif&lineHeight=34&lang=ko&method=span&requestId=debug1

The page shows pretext's predicted lines vs the browser's actual lines side by side. On main (before this fix), it will show a mismatch.

Fix

// src/analysis.ts — isCJKCodePoint()
+    (codePoint >= 0x3130 && codePoint <= 0x318F) ||  // Hangul Compatibility Jamo (ㄱ-ㅣ)
     (codePoint >= 0xAC00 && codePoint <= 0xD7AF) ||

Without this range, ㅋㅋㅋ is treated as a single word unit rather than being split into individual CJK grapheme units. The line-breaker tries to fit the entire run as one piece, while browsers allow breaks between consecutive Compatibility Jamo at word boundaries — the same way they handle other CJK characters.

Verification

All existing tests pass (bun test).

Added scripts/korean-check.ts — a dedicated oracle checker (19 cases) following the keep-all-check.ts pattern.

After fix — Chrome:

Korean Layout Check — Chrome
────────────────────────────────────────────────────────────
  ✓ PASS  B1: Hangul Jamo standalone (U+1100)             [3 lines]
  ✓ PASS  B2: Hangul Compatibility Jamo (U+3130)          [4 lines]
  ✓ PASS  B3: Korean+English mixed                        [3 lines]
  ✓ PASS  B4: Korean+numbers mixed                        [4 lines]
  ✓ PASS  B5: Korean+CJK punctuation                      [4 lines]
  ✓ PASS  B6: NBSP + Korean                               [3 lines]
  ✓ PASS  B2c-w160: ㅠㅠ crying expression (160px)        [4 lines]
  ✓ PASS  B2c-w140: ㅠㅠ crying expression (140px)        [5 lines]
  ✓ PASS  B2d-w150: ㄹㅇ literally slang (150px)          [4 lines]
  ✓ PASS  B2f-w150: ㅇㅋ/ㄴㄴ okay/nope slang (150px)     [5 lines]
  ✓ PASS  B2b: ㅋㅋ laughter slang mixed                  [3 lines]
  ✓ PASS  B2c: ㅠㅠ crying expression                     [3 lines]
  ✓ PASS  B2d: ㄹㅇ literally slang mid-sentence          [3 lines]
  ✓ PASS  B2e: consonants-only run                        [5 lines]
  ✓ PASS  B2f: ㅇㅋ/ㄴㄴ okay/nope internet slang         [4 lines]
  ✓ PASS  C1: keep-all + narrow width                     [9 lines]
  ✓ PASS  C2: keep-all + Korean+English mixed             [4 lines]
  ✓ PASS  C3: pre-wrap + Korean hard break                [2 lines]
  ✓ PASS  C4: pre-wrap + tab + Korean                     [1 lines]

Summary: chrome 19/19 pass

After fix — Safari:

Korean Layout Check — Safari
────────────────────────────────────────────────────────────
  ✓ PASS  B1: Hangul Jamo standalone (U+1100)             [3 lines]
  ✓ PASS  B2: Hangul Compatibility Jamo (U+3130)          [4 lines]
  ✓ PASS  B3: Korean+English mixed                        [3 lines]
  ✓ PASS  B4: Korean+numbers mixed                        [4 lines]
  ✓ PASS  B5: Korean+CJK punctuation                      [4 lines]
  ✓ PASS  B6: NBSP + Korean                               [3 lines]
  ✓ PASS  B2c-w160: ㅠㅠ crying expression (160px)        [4 lines]
  ✓ PASS  B2c-w140: ㅠㅠ crying expression (140px)        [5 lines]
  ✓ PASS  B2d-w150: ㄹㅇ literally slang (150px)          [4 lines]
  ✓ PASS  B2f-w150: ㅇㅋ/ㄴㄴ okay/nope slang (150px)     [5 lines]
  ✓ PASS  B2b: ㅋㅋ laughter slang mixed                  [3 lines]
  ✓ PASS  B2c: ㅠㅠ crying expression                     [3 lines]
  ✓ PASS  B2d: ㄹㅇ literally slang mid-sentence          [3 lines]
  ✓ PASS  B2e: consonants-only run                        [5 lines]
  ✓ PASS  B2f: ㅇㅋ/ㄴㄴ okay/nope internet slang         [4 lines]
  ✓ PASS  C1: keep-all + narrow width                     [9 lines]
  ✓ PASS  C2: keep-all + Korean+English mixed             [4 lines]
  ✓ PASS  C3: pre-wrap + Korean hard break                [2 lines]
  ✓ PASS  C4: pre-wrap + tab + Korean                     [1 lines]

Summary: safari 19/19 pass

[chenglou]

Thanks! I've implement this and credited you as co-author

[chenglou]

@mayrang got your message! Prev commits formatting screwed up. Fixed!

[mayrang]

Awesome, thanks for fixing it! Really appreciate your help. Have a great weekend.