fibers B1.0a: close generic/pack is_pure gap (review)

Adversarial review of dd363ca found is_pure was set only at the two
declareFunction decl sites. Generic monomorphization (generic.zig) and
pack expansion (pack.zig) create the IR Function via a different path
and left is_pure false, so a generic abi(.pure) instance bypassed the
emit bail and silently shipped a framed body — it returned 42 but
leaked the prologue's stack adjustment (the exact SP-in != SP-out
corruption the lock exists to prevent).

Both paths now set is_pure and route .pure bodies through the asm-only
+ unreachable cap, mirroring the decl path. Locked by
examples/1801-concurrency-pure-generic-bail.sx (generic .pure reaches
the loud bail).

The review's other CRITICAL (a .pure lambda) is a false positive:
isLambda's return-type scan (parser.zig:3652) breaks on the abi
keyword, so a .pure lambda is unparseable and parseLambda's abi
handling is never reached. Latent isLambda/parseLambda inconsistency,
not a B1 concern.

Suite green (723/0).

src/ir/lower/generic.zig

@@ -115,6 +115,7 @@ pub fn monomorphizeFunction(self: *Lowering, fd: *const ast.FnDecl, mangled_name
     const func_id = self.builder.beginFunction(name_id, params.items, ret_ty);
     _ = func_id;
     self.builder.currentFunc().has_implicit_ctx = wants_ctx;
+    self.builder.currentFunc().is_pure = (fd.abi == .pure);
 
     // Create entry block
     const entry_name = self.module.types.internString("entry");
@@ -151,6 +152,13 @@ pub fn monomorphizeFunction(self: *Lowering, fd: *const ast.FnDecl, mangled_name
             self.ensureTerminator(ret_ty);
         }
         self.builder.finalize();
+    } else if (self.builder.currentFunc().is_pure) {
+        // `abi(.pure)`: asm-only body that rets itself — no sx value return.
+        // Lower the statements + cap with `unreachable` (mirrors the decl path).
+        // emit_llvm bails on `is_pure` until B1.0b implements `naked` emission.
+        self.lowerBlock(fd.body);
+        if (!self.currentBlockHasTerminator()) self.builder.emitUnreachable();
+        self.builder.finalize();
     } else {
         // Lower the function body
         if (ret_ty != .void) {