library: vendors/stb_image + vendors/stb_truetype — stb ships with sx

The stb headers move from the repo-root vendors/ (resolvable only
with CWD = sx repo) into library/vendors/ following the sqlite
convention — bindings module + c/ sources + provenance README — so
'#import "vendors/stb_image/stb_image.sx"' (image v2.30 + image_write
v1.16) and '#import "vendors/stb_truetype/stb_truetype.sx"' (v1.26)
work from any consumer via the stdlib search paths. modules/ffi/stb.sx
dissolves into the stb_image vendor; modules/ffi/stb_truetype.sx keeps
its non-stb text-shaping companions and re-imports the vendored unit.
examples/1625 pins a deterministic in-memory BMP decode; examples/1626
pins font init + metric invariants against the system Helvetica.

examples/1603-platform-stb-image.sx

@@ -1,5 +1,5 @@
 #import "modules/std.sx";
-stb :: #import "modules/ffi/stb.sx";
+stb :: #import "vendors/stb_image/stb_image.sx";
 
 main :: () -> i32 {
     w: i32 = 0;

examples/1625-vendor-stb-image-decode.sx

@@ -0,0 +1,35 @@
+// The sx library ships stb_image: `#import "vendors/stb_image/
+// stb_image.sx"` resolves through the stdlib search paths and the
+// implementation compiles through the object cache. Decodes a 2x2
+// 24-bit BMP built in memory — fully deterministic: dimensions,
+// channel count, and the top-left pixel (blue) are pinned.
+#import "modules/std.sx";
+stb :: #import "vendors/stb_image/stb_image.sx";
+
+main :: () -> i32 {
+    // BITMAPFILEHEADER (14) + BITMAPINFOHEADER (40) + 2 rows of
+    // 2 BGR pixels padded to 4-byte rows (8 each) = 70 bytes.
+    // Bottom row: red, green; top row: blue, white (BMP is bottom-up).
+    bmp : [70]u8 = .{
+        66, 77, 70, 0, 0, 0, 0, 0, 0, 0, 54, 0, 0, 0,
+        40, 0, 0, 0, 2, 0, 0, 0, 2, 0, 0, 0, 1, 0, 24, 0,
+        0, 0, 0, 0, 16, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0, 0, 0, 0,
+        0, 0, 255, 0, 255, 0, 0, 0,
+        255, 0, 0, 255, 255, 255, 0, 0,
+    };
+
+    w : i32 = 0;
+    h : i32 = 0;
+    ch : i32 = 0;
+    img := stb.stbi_load_from_memory(@bmp[0], 70, @w, @h, @ch, 3);
+    if xx img == 0 {
+        print("decode failed\n");
+        return 1;
+    }
+    p : [*]u8 = xx img;
+    print("decoded {}x{} ({} channels)\n", w, h, ch);
+    print("top-left pixel: {} {} {}\n", p[0], p[1], p[2]);
+    stb.stbi_image_free(xx img);
+    0
+}

examples/1626-vendor-stb-truetype-metrics.sx

@@ -0,0 +1,36 @@
+// The sx library ships stb_truetype: `#import "vendors/stb_truetype/
+// stb_truetype.sx"` resolves through the stdlib search paths and the
+// implementation compiles through the object cache. Loads the system
+// Helvetica collection and pins INVARIANTS only (font versions vary
+// across macOS releases): init succeeds, the pixel-height scale is
+// positive, the ascender is positive and the descender negative.
+#import "modules/std.sx";
+fs :: #import "modules/std/fs.sx";
+tt :: #import "vendors/stb_truetype/stb_truetype.sx";
+
+main :: () -> i32 {
+    data := fs.read_file("/System/Library/Fonts/Helvetica.ttc");
+    if data == null {
+        print("font missing\n");
+        return 1;
+    }
+    bytes := data!;
+
+    off := tt.stbtt_GetFontOffsetForIndex(bytes.ptr, 0);
+    print("font offset >= 0: {}\n", off >= 0);
+
+    info : *void = xx context.allocator.alloc_bytes(256);
+    ok := tt.stbtt_InitFont(info, bytes.ptr, off);
+    print("init ok: {}\n", ok != 0);
+
+    px : f32 = 32.0;
+    scale := tt.stbtt_ScaleForPixelHeight(info, px);
+    print("scale > 0: {}\n", scale > 0.0);
+
+    ascent : i32 = 0;
+    descent : i32 = 0;
+    linegap : i32 = 0;
+    tt.stbtt_GetFontVMetrics(info, @ascent, @descent, @linegap);
+    print("ascent > 0, descent < 0: {} {}\n", ascent > 0, descent < 0);
+    0
+}

examples/expected/1625-vendor-stb-image-decode.exit

@@ -0,0 +1 @@
+0

examples/expected/1625-vendor-stb-image-decode.stdout

@@ -0,0 +1,2 @@
+decoded 2x2 (3 channels)
+top-left pixel: 0 0 255

examples/expected/1626-vendor-stb-truetype-metrics.exit

@@ -0,0 +1 @@
+0

examples/expected/1626-vendor-stb-truetype-metrics.stdout

@@ -0,0 +1,4 @@
+font offset >= 0: true
+init ok: true
+scale > 0: true
+ascent > 0, descent < 0: true true