fix(css): preserve logical border-radius properties (#26006)

## Summary
- CSS logical border-radius properties (`border-start-start-radius`,
`border-start-end-radius`, `border-end-end-radius`,
`border-end-start-radius`) were being silently dropped when processed by
the CSS bundler
- The bug was in `src/css/properties/border_radius.zig` where
`VendorPrefix{}` (all fields false) was used instead of `VendorPrefix{
.none = true }` when computing prefixes for logical properties
- This caused the properties to be dropped by a later `isEmpty()` check
since an empty prefix struct was returned

## Test plan
- [x] Added regression test `test/regression/issue/25785.test.ts`
- [x] Verified test fails with system Bun (`USE_SYSTEM_BUN=1 bun test`)
- [x] Verified test passes with fixed bun-debug (`bun bd test`)

Fixes #25785

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>

src/css/properties/border_radius.zig

@@ -220,7 +220,7 @@ pub const BorderRadiusHandler = struct {
             if (logical_supported) {
                 bun.handleOom(d.append(ctx.allocator, v));
             } else {
-                const prefix = ctx.targets.prefixes(css.VendorPrefix{}, css.prefixes.Feature.border_radius);
+                const prefix = ctx.targets.prefixes(css.VendorPrefix{ .none = true }, css.prefixes.Feature.border_radius);
                 switch (v) {
                     .@"border-start-start-radius",
                     .@"border-start-end-radius",

test/regression/issue/25785.test.ts

@@ -0,0 +1,106 @@
+import { expect, test } from "bun:test";
+import { tempDir } from "harness";
+
+// Regression test for https://github.com/oven-sh/bun/issues/25785
+// CSS logical border-radius properties were being silently dropped
+
+test("CSS bundler should preserve logical border-radius properties", async () => {
+  using dir = tempDir("issue-25785", {
+    "test.css": `
+.test1 {
+  border-start-start-radius: 0.75rem;
+}
+.test2 {
+  border-end-start-radius: 0.75rem;
+}
+.test3 {
+  border-start-end-radius: 0.75rem;
+}
+.test4 {
+  border-end-end-radius: 0.75rem;
+}
+.test5 {
+  border-top-left-radius: 0.75rem;
+}
+`,
+  });
+
+  const result = await Bun.build({
+    entrypoints: [`${dir}/test.css`],
+    outdir: `${dir}/dist`,
+    experimentalCss: true,
+    minify: false,
+  });
+
+  expect(result.success).toBe(true);
+  expect(result.outputs.length).toBe(1);
+
+  const output = await result.outputs[0].text();
+
+  // Logical properties are compiled to physical properties with LTR/RTL rules
+  // .test1 with border-start-start-radius compiles to border-top-left-radius (LTR) and border-top-right-radius (RTL)
+  expect(output).toContain(".test1");
+  expect(output).toContain("border-top-left-radius");
+  expect(output).toContain("border-top-right-radius");
+
+  // .test2 with border-end-start-radius compiles to border-bottom-left-radius (LTR) and border-bottom-right-radius (RTL)
+  expect(output).toContain(".test2");
+  expect(output).toContain("border-bottom-left-radius");
+  expect(output).toContain("border-bottom-right-radius");
+
+  // .test3 with border-start-end-radius
+  expect(output).toContain(".test3");
+
+  // .test4 with border-end-end-radius
+  expect(output).toContain(".test4");
+
+  // Physical property should also be preserved
+  expect(output).toContain(".test5");
+});
+
+test("CSS bundler should handle logical border-radius with targets that compile logical properties", async () => {
+  using dir = tempDir("issue-25785-compiled", {
+    "test.css": `
+.test1 {
+  border-start-start-radius: 0.75rem;
+}
+.test2 {
+  border-end-start-radius: 0.75rem;
+}
+.test3 {
+  border-start-end-radius: 0.75rem;
+}
+.test4 {
+  border-end-end-radius: 0.75rem;
+}
+`,
+  });
+
+  const result = await Bun.build({
+    entrypoints: [`${dir}/test.css`],
+    outdir: `${dir}/dist`,
+    experimentalCss: true,
+    minify: false,
+    // Target older browsers that don't support logical properties
+    target: "browser",
+  });
+
+  expect(result.success).toBe(true);
+  expect(result.outputs.length).toBe(1);
+
+  const output = await result.outputs[0].text();
+
+  // When logical properties are compiled down, they should produce physical properties
+  // with :lang() selectors to handle LTR/RTL
+  // At minimum, the output should NOT be empty (the bug caused empty output)
+  expect(output.trim().length).toBeGreaterThan(0);
+
+  // Should have some border-radius output (compiled to physical)
+  expect(output).toMatch(/border-.*-radius/);
+
+  // All classes should be present in the output
+  expect(output).toContain(".test1");
+  expect(output).toContain(".test2");
+  expect(output).toContain(".test3");
+  expect(output).toContain(".test4");
+});