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Replace Bun's outdated grapheme breaking implementation with [Ghostty's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>

src/string/immutable/visible.zig

@@ -927,7 +927,7 @@ pub const visible = struct {
         var input = input_;
         var len: usize = 0;
         var prev: ?u32 = null;
-        var break_state = grapheme.BreakState{};
+        var break_state: grapheme.BreakState = .default;
         var grapheme_state = GraphemeState{};
         var saw_1b = false;
         var saw_csi = false; // CSI: ESC [
@@ -964,7 +964,7 @@ pub const visible = struct {
                         const last_cp: u32 = input[bulk_end - 1];
                         grapheme_state.reset(last_cp, ambiguousAsWide);
                         prev = last_cp;
-                        break_state = grapheme.BreakState{};
+                        break_state = .default;
 
                         // If we consumed everything, advance and continue
                         if (bulk_end == idx) {