fibers B1.0c: support params in abi(.pure) (read from registers)

Adversarial review of B1.0b found a param-bearing abi(.pure) function
emitted invalid LLVM ("cannot use argument of naked function" — loud
verifier error, not silent) because the param-alloca loop spilled the
args to stack slots, which a naked function cannot have.

Fixed forward — this ENABLES the B1.3 context-switch use case rather
than rejecting it: gate the param-alloca loop on fd.abi != .pure in
decl.zig (both body-lowering paths) and generic.zig. A naked function's
args stay in their ABI registers and are read directly by the asm body
(e.g. swap_context reads from/to from x0/x1); the LLVM args are
declared-but-unused, which the verifier allows.

examples/1803-concurrency-pure-asm-param.sx: naked add(a, b) reads x0/x1
(add x0, x0, x1; ret) -> 40 + 2 = 42. aarch64-pinned.

Pack abi(.pure) (variadic + naked — nonsensical, can't read a runtime
pack from registers) left unsupported: pack.zig's param loop is
intertwined with comptime-param/#insert handling, so that case still
hits the loud verifier error. Documented in the checkpoint.

Also updates PLAN-FIBERS / CHECKPOINT-FIBERS for B1.0 completion.
B1.0 complete. Suite green (725/0).

examples/expected/1803-concurrency-pure-asm-param.build

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

examples/expected/1803-concurrency-pure-asm-param.exit

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

examples/expected/1803-concurrency-pure-asm-param.ir

@@ -0,0 +1,15 @@
+
+; Function Attrs: naked noinline nounwind
+define internal i64 @add(i64 %0, i64 %1) #0 {
+entry:
+  call void asm sideeffect "        add x0, x0, x1\0A        ret\0A", ""()
+  unreachable
+}
+
+; Function Attrs: nounwind
+define i32 @main() #1 {
+entry:
+  %call = call i64 @add(i64 40, i64 2)
+  %ca.tr = trunc i64 %call to i32
+  ret i32 %ca.tr
+}

examples/expected/1803-concurrency-pure-asm-param.stderr

@@ -0,0 +1 @@
+

examples/expected/1803-concurrency-pure-asm-param.stdout

@@ -0,0 +1 @@
+