fix: preserve FP control state across SysV context switch, expand tests

CHANGELOG.md

@@ -12,6 +12,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `examples/basic.zig` and `examples/scheduler.zig`, runnable via
   `zig build examples`.
 - GitHub Actions CI building and testing on Linux and Windows (x86_64).
+- Expanded test coverage: cross-platform FP control-state preservation, nested
+  fibers, deep-stack yields, many yields, local-variable integrity, interleaved
+  independent fibers, and allocation-failure cleanup.
+
+### Fixed
+
+- The SysV (Linux/BSD) context switch now preserves the MXCSR control bits and
+  the x87 control word across a switch, matching the Windows path and the SysV
+  ABI. Previously a fiber that changed the rounding mode or FP exception masks
+  could leak that state into whatever ran next.
 
 ### Removed
 

src/arch/x86_64_sysv.zig

@@ -8,8 +8,17 @@ fn fiberSwapNaked() callconv(.naked) void {
         \\  pushq %%r13
         \\  pushq %%r14
         \\  pushq %%r15
+        \\  // MXCSR (+0) and x87 control word (+8) in a 16-byte slot
+        \\  subq $16, %%rsp
+        \\  stmxcsr 0(%%rsp)
+        \\  fnstcw 8(%%rsp)
+        \\  // switch stacks
         \\  movq %%rsp, (%%rdi)
         \\  movq (%%rsi), %%rsp
+        \\  // restore MXCSR + x87 control word
+        \\  ldmxcsr 0(%%rsp)
+        \\  fldcw 8(%%rsp)
+        \\  addq $16, %%rsp
         \\  popq %%r15
         \\  popq %%r14
         \\  popq %%r13
@@ -46,5 +55,12 @@ pub fn initStack(
         @as(*usize, @ptrFromInt(sp)).* = 0;
     }
 
+    // MXCSR (+0) / x87 control word (+8) 16-byte slot with defaults, matching
+    // the Windows layout so the first swap-in loads a sane FP environment.
+    sp -= @sizeOf(usize);
+    @as(*usize, @ptrFromInt(sp)).* = 0x0000037F; // x87 control word (slot+8)
+    sp -= @sizeOf(usize);
+    @as(*usize, @ptrFromInt(sp)).* = 0x00001F80; // MXCSR (slot+0)
+
     return sp;
 }