fibers B1.0b: abi(.pure) emits a real LLVM naked function (green)

Flip the B1.0a emit bail to real emission. The emit_llvm declaration
pass now adds LLVM's naked + noinline + nounwind attributes for an
is_pure function and skips frame-pointer=all (incompatible with a
frameless function); Pass 2 emits the body normally, and the naked
attribute makes the backend emit it verbatim (the inline asm + its own
ret) with no prologue/epilogue.

IR shape verified:
  ; Function Attrs: naked noinline nounwind
  define internal i64 @answer() #0 {
  entry:
    call void asm sideeffect "...ret...", ""()
    unreachable
  }
The caller invokes it as an ordinary () -> i64 call (.pure is
call_conv == .default).

- examples/1800-concurrency-pure-asm.sx: now green, aarch64-pinned
  (.build macos) -> exit 42 + .ir snapshot.
- examples/1801-concurrency-pure-generic.sx (renamed from -bail): the
  generic .pure now emits a correct naked answer__i64 (exit 42),
  proving generic.zig produces a naked body, not a framed one.
- examples/1802-concurrency-pure-asm-x86.sx: x86_64 cross sibling
  (.build x86_64-linux, ir-only here); .ir locks naked + movl $42,%eax.
- unit test in emit_llvm.test.zig asserts the naked attribute is present
  and frame-pointer absent on an abi(.pure) function.

Suite green (724/0).

examples/expected/1800-concurrency-pure-asm.build

@@ -0,0 +1 @@
+{ "target": "macos" }

examples/expected/1800-concurrency-pure-asm.exit

@@ -1 +1 @@
-1
+42

examples/expected/1800-concurrency-pure-asm.ir

@@ -0,0 +1,15 @@
+
+; Function Attrs: naked noinline nounwind
+define internal i64 @answer() #0 {
+entry:
+  call void asm sideeffect "        mov x0, #42\0A        ret\0A", ""()
+  unreachable
+}
+
+; Function Attrs: nounwind
+define i32 @main() #1 {
+entry:
+  %call = call i64 @answer()
+  %ca.tr = trunc i64 %call to i32
+  ret i32 %ca.tr
+}

examples/expected/1800-concurrency-pure-asm.stderr

@@ -1 +1 @@
-error: `abi(.pure)` function 'answer' LLVM emission not yet implemented
+

examples/expected/1801-concurrency-pure-generic-bail.exit

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

examples/expected/1801-concurrency-pure-generic-bail.stderr

@@ -1 +0,0 @@
-error: `abi(.pure)` function 'answer__i64' LLVM emission not yet implemented

examples/expected/1801-concurrency-pure-generic.build

@@ -0,0 +1 @@
+{ "target": "macos" }

examples/expected/1801-concurrency-pure-generic.exit

@@ -0,0 +1 @@
+42

examples/expected/1801-concurrency-pure-generic.ir

@@ -0,0 +1,15 @@
+
+; Function Attrs: nounwind
+define i32 @main() #0 {
+entry:
+  %call = call i64 @answer__i64()
+  %ca.tr = trunc i64 %call to i32
+  ret i32 %ca.tr
+}
+
+; Function Attrs: naked noinline nounwind
+define internal i64 @answer__i64() #1 {
+entry:
+  call void asm sideeffect "        mov x0, #42\0A        ret\0A", ""()
+  unreachable
+}

examples/expected/1801-concurrency-pure-generic.stdout

@@ -0,0 +1 @@
+

examples/expected/1802-concurrency-pure-asm-x86.build

@@ -0,0 +1 @@
+{ "target": "x86_64-linux" }

examples/expected/1802-concurrency-pure-asm-x86.exit

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

examples/expected/1802-concurrency-pure-asm-x86.ir

@@ -0,0 +1,15 @@
+
+; Function Attrs: naked noinline nounwind
+define internal i64 @answer() #0 {
+entry:
+  call void asm sideeffect "        movl $$42, %eax\0A        ret\0A", ""()
+  unreachable
+}
+
+; Function Attrs: nounwind
+define i32 @main() #1 {
+entry:
+  %call = call i64 @answer()
+  %ca.tr = trunc i64 %call to i32
+  ret i32 %ca.tr
+}

examples/expected/1802-concurrency-pure-asm-x86.stderr

@@ -0,0 +1 @@
+