fibers B1.0a: plumb abi(.pure), emit bails (lock)

First implementation step of Stream B1 (fibers). Make the inert abi(.pure)
ABI carry an is_pure flag through lowering, with LLVM emission deliberately
bailing loudly until B1.0b — the lock half of the lock->green cadence.

- IR Function.is_pure, set from fd.abi == .pure at both declareFunction
  decl sites.
- funcWantsImplicitCtx skips .pure (no synthetic __sx_ctx, mirroring the
  .c skip): a pure fn reads args from ABI registers, an implicit ctx would
  occupy a register slot the asm doesn't expect.
- both body-lowering paths bypass lowerValueBody for .pure: lower the asm
  body as statements + cap with unreachable. A pure body has no sx return
  (the asm rets itself), so the implicit-return diagnostic must not fire.
- emit_llvm Pass 2 bails loudly when func.is_pure (build-gating nonzero
  exit) rather than emit a framed body, whose epilogue would corrupt a
  context switch's deliberate SP-in != SP-out.

examples/1800-concurrency-pure-asm.sx: one host example (no .build pin --
the bail fires before instruction selection, so it is host-independent),
locked to the bail snapshot. B1.0b flips emit to LLVM's naked attribute +
asm-only body and pins the example per-arch.

The sx-facing name is "pure" throughout (field, diagnostic); LLVM's naked
attribute is only the B1.0b lowering mechanism. Suite green (722/0).

src/ir/inst.zig

@@ -640,6 +640,16 @@ pub const Function = struct {
     /// drops the leftover declaration. See current/PLAN-COMPILER-VM.md (S3).
     is_compiler_domain: bool = false,
 
+    /// True for an `abi(.pure)` function — no calling-convention
+    /// prologue/epilogue/frame, no implicit `__sx_ctx`. Its body is a single
+    /// inline-asm block that reads args from ABI registers and emits its own
+    /// `ret` (the context-switch primitive; design §4.6). emit_llvm lowers this
+    /// via LLVM's `naked` function attribute and generates no frame setup. A
+    /// `.c` epilogue would restore SP from the wrong stack across a context
+    /// switch (SP-in ≠ SP-out by design), which is why `.pure` is distinct
+    /// from `.c`.
+    is_pure: bool = false,
+
     pub const Param = struct {
         name: StringId,
         ty: TypeId,