feat(unicode): migrate grapheme breaking to uucode with GB9c support (#26376)

## Summary

Replace Bun's outdated grapheme breaking implementation with [Ghostty's
approach](https://github.com/ghostty-org/ghostty/tree/main/src/unicode)
using the [uucode](https://github.com/jacobsandlund/uucode) library.
This adds proper **GB9c (Indic Conjunct Break)** support — Devanagari
and other Indic script conjuncts now correctly form single grapheme
clusters.

## Motivation

The previous implementation used a `GraphemeBoundaryClass` enum with
only 12 values and a 2-bit `BreakState` (just `extended_pictographic`
and `regional_indicator` flags). It had no support for Unicode's GB9c
rule, meaning Indic conjunct sequences (consonant + virama + consonant)
were incorrectly split into multiple grapheme clusters.

## Architecture

### Runtime (zero uucode dependency, two table lookups)

```
codepoint → [3-level LUT] → GraphemeBreakNoControl enum (u5, 17 values)
(state, gb1, gb2) → [8KB precomputed array] → (break_result, new_state)
```

The full grapheme break algorithm (GB6-GB13, GB9c, GB11, GB999) runs
only at **comptime** to populate the 8KB decision array. At runtime it's
pure table lookups.

### File Layout

```
src/deps/uucode/              ← Vendored library (MIT, build-time only)
src/unicode/uucode/           ← Build-time integration
  ├── uucode_config.zig       ← What Unicode properties to generate
  ├── grapheme_gen.zig        ← Generator: queries uucode → writes tables
  ├── lut.zig                 ← 3-level lookup table generator
  └── CLAUDE.md               ← Maintenance docs
src/string/immutable/         ← Runtime (no uucode dependency)
  ├── grapheme.zig            ← Grapheme break API + comptime decisions
  ├── grapheme_tables.zig     ← Pre-generated tables (committed, ~91KB source)
  └── visible.zig             ← Width calculation (2 lines changed)
scripts/update-uucode.sh      ← Update vendored uucode + regenerate
```

### Key Types

| Type | Size | Values |
|------|------|--------|
| `GraphemeBreakNoControl` | u5 | 17 (adds
`indic_conjunct_break_{consonant,linker,extend}`, `emoji_modifier_base`,
`zwnj`, etc.) |
| `BreakState` | u3 | 5 (`default`, `regional_indicator`,
`extended_pictographic`, `indic_conjunct_break_consonant`,
`indic_conjunct_break_linker`) |

### Binary Size

The tables store only the `GraphemeBreakNoControl` enum per codepoint
(not width or emoji properties, which visible.zig handles separately):

- stage1: 8192 × u16 = **16KB** (maps high byte → stage2 offset)
- stage2: 27392 × u8 = **27KB** (maps to stage3 index; max value is 16)
- stage3: 17 × u5 = **~17 bytes** (one per enum value)
- Precomputed decisions: **8KB**
- **Total: ~51KB** (vs previous ~70KB+)

## How to Regenerate Tables

```bash
# After updating src/deps/uucode/:
./scripts/update-uucode.sh

# Or manually:
vendor/zig/zig build generate-grapheme-tables
```

Normal builds never run the generator — they use the committed
`grapheme_tables.zig`.

## Testing

```bash
bun bd test test/js/bun/util/stringWidth.test.ts
```

New test cases verify Devanagari conjuncts (GB9c):
- `क्ष` (Ka+Virama+Ssa) → single cluster, width 2
- `क्‍ष` (Ka+Virama+ZWJ+Ssa) → single cluster, width 2
- `क्क्क` (Ka+Virama+Ka+Virama+Ka) → single cluster, width 3

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

scripts/update-uucode.sh

@@ -0,0 +1,82 @@
+#!/bin/bash
+# Updates the vendored uucode library and regenerates grapheme tables.
+#
+# Usage:
+#   ./scripts/update-uucode.sh                    # update from default URL
+#   ./scripts/update-uucode.sh /path/to/uucode    # update from local directory
+#   ./scripts/update-uucode.sh https://url.tar.gz # update from URL
+#
+# After running, verify with:
+#   bun bd test test/js/bun/util/stringWidth.test.ts
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+BUN_ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
+UUCODE_DIR="$BUN_ROOT/src/deps/uucode"
+ZIG="$BUN_ROOT/vendor/zig/zig"
+
+if [ ! -x "$ZIG" ]; then
+    echo "error: zig not found at $ZIG"
+    echo "       run scripts/bootstrap.sh first"
+    exit 1
+fi
+
+update_from_dir() {
+    local src="$1"
+    echo "Updating uucode from: $src"
+    rm -rf "$UUCODE_DIR"
+    mkdir -p "$UUCODE_DIR"
+    cp -r "$src"/* "$UUCODE_DIR/"
+}
+
+update_from_url() {
+    local url="$1"
+    local tmp
+    tmp=$(mktemp -d)
+    trap "rm -rf $tmp" EXIT
+
+    echo "Downloading uucode from: $url"
+    curl -fsSL "$url" | tar -xz -C "$tmp" --strip-components=1
+
+    update_from_dir "$tmp"
+}
+
+# Handle source argument
+if [ $# -ge 1 ]; then
+    SOURCE="$1"
+    if [ -d "$SOURCE" ]; then
+        update_from_dir "$SOURCE"
+    elif [[ "$SOURCE" == http* ]]; then
+        update_from_url "$SOURCE"
+    else
+        echo "error: argument must be a directory or URL"
+        exit 1
+    fi
+else
+    # Default: use the zig global cache if available
+    CACHED=$(find "$HOME/.cache/zig/p" -maxdepth 1 -name "uucode-*" -type d 2>/dev/null | sort -V | tail -1)
+    if [ -n "$CACHED" ]; then
+        update_from_dir "$CACHED"
+    else
+        echo "error: no uucode source specified and none found in zig cache"
+        echo ""
+        echo "usage: $0 <path-to-uucode-dir-or-url>"
+        exit 1
+    fi
+fi
+
+echo ""
+echo "Regenerating grapheme tables..."
+cd "$BUN_ROOT"
+"$ZIG" build generate-grapheme-tables
+
+echo ""
+echo "Done. Updated files:"
+echo "  src/deps/uucode/         (vendored library)"
+echo "  src/string/immutable/grapheme_tables.zig (regenerated)"
+echo ""
+echo "Next steps:"
+echo "  1. bun bd test test/js/bun/util/stringWidth.test.ts"
+echo "  2. git add src/deps/uucode src/string/immutable/grapheme_tables.zig"
+echo "  3. git commit -m 'Update uucode to <version>'"