libghostty: faster render state reads and updates on wasm targets

This makes the `ghostty_render_state_*` C API significantly faster on
wasm32-freestanding, measured in V8 via Node for Chrome. Also verified
in `jsc` for Safari.

The major change is a new bulk row read API that makes full-screen cell reads
roughly 10x faster for wasm embedders. This should help any embedder with
high FFI overhead, such as Go, Python, etc. too.

Non-wasm performance is not impacted, all benchmarks were run on my mac
too w/ no regressions (two of the changes are native wins as well).

## Changes

* color: the "vectorized" palette conversion loop was silently
  scalarized by LLVM into per-byte ops because it loaded/stored through
  array-typed pointers. Zig 0.16 disables the LLVM loop vectorizer, so
  manually vectorized loops must go through vector-typed pointers.
* C styles: major optimizations to converting Zig styles to C styles.
  This is a heavy operation for render state.
* render: `endUpdate`'s style-run fill (`@memset` with a struct value)
  re-loaded its source every iteration and stored field by field. Now
  manually vectorized.
* render: new `GHOSTTY_RENDER_STATE_ROW_DATA_CELLS_RAW` returns a
  borrowed `GhosttyCellsView` of the current row's raw cell values, valid
  until the next update. One call per row instead of 3-6 calls per cell.

## Benchmarks

| Benchmark | Before | After | Speedup |
|---|---|---|---|
| colors_get | 114 ns | 35 ns | 3.3x |
| style get, per styled cell | 7.8 ns | 6.7 ns | 1.2x |
| raw+style read, per cell | 8.6 ns | 7.7 ns | 1.1x |
| full-screen text read, per cell | 7.5 ns | 0.7 ns | 10.7x |
| full-screen text+style read, per cell | 8.6 ns | 1.7 ns | 5.1x |
| render state update, styled full frame | 3.4 us | 2.6 us | 1.3x |

**AI usage:** Fable did the implementation and benchmarking and drafted
this message. Comments were partially rewritten by me.

include/ghostty/vt/render.h

@@ -236,6 +236,19 @@ typedef enum GHOSTTY_ENUM_TYPED {
 
   /** Row-local selected cell range (GhosttyRenderStateRowSelection). */
   GHOSTTY_RENDER_STATE_ROW_DATA_SELECTION = 4,
+
+  /** A borrowed view of the raw cell values for the current row
+   *  (GhosttyCellsView). One value per column, identical to querying
+   *  GHOSTTY_RENDER_STATE_ROW_CELLS_DATA_RAW for each cell. The view
+   *  is only valid as long as the underlying render state is not
+   *  updated; it is unsafe to use after updating the render state.
+   *
+   *  This is the bulk alternative to iterating cells one at a time.
+   *  It lets callers with expensive call boundaries (e.g. WebAssembly
+   *  embedders) read an entire row with a single call, then drill
+   *  into the cells iterator only for cells that need managed data
+   *  (styles, graphemes). */
+  GHOSTTY_RENDER_STATE_ROW_DATA_CELLS_RAW = 5,
   GHOSTTY_RENDER_STATE_ROW_DATA_MAX_VALUE = GHOSTTY_ENUM_MAX_VALUE,
 } GhosttyRenderStateRowData;
 

include/ghostty/vt/screen.h

@@ -50,6 +50,23 @@ typedef uint64_t GhosttyCell;
  */
 typedef uint64_t GhosttyRow;
 
+/**
+ * A borrowed view of contiguous raw cell values.
+ *
+ * The memory is not owned by this struct. The pointer is only valid
+ * for the lifetime documented by the API that produces it. Each value
+ * is queried via ghostty_cell_get() like any other GhosttyCell.
+ *
+ * @ingroup screen
+ */
+typedef struct {
+  /** Pointer to len contiguous cell values. */
+  const GhosttyCell* ptr;
+
+  /** Number of cells. */
+  size_t len;
+} GhosttyCellsView;
+
 /**
  * Cell content tag.
  *