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).

current/PLAN-FIBERS.md

@@ -166,7 +166,7 @@ B1.0 (`.pure`) forces these plumbing sites:
   emit the `.pure` body as the asm block only (no prologue/epilogue/ctx). Pin `1800`
   aarch64 (`.build {"target":"aarch64-macos"}`) → runs end-to-end (exit 42) on this host,
   ir-only on a mismatch; capture its `.ir` (asserts `naked` + the asm). Add an x86_64 cross
-  sibling `examples/1801-concurrency-pure-asm-x86.sx` (`.build {"target":"x86_64-linux"}`,
+  sibling `examples/1802-concurrency-pure-asm-x86.sx` (`.build {"target":"x86_64-linux"}`,
   ir-only here). Add a unit test in `emit_llvm.test.zig` asserting the `naked` attribute is
   present on a `.pure` function. Review the diff (no stray error text). Commit.
 
@@ -232,7 +232,7 @@ asserting program-emitted ordering contracts.
 > `{"target":"aarch64-macos"}`; on this aarch64 host it runs end-to-end (exit 42), capture
 > `.ir` + regen (`-Dname=examples/1800-concurrency-pure-asm.sx -Dupdate-goldens`), review the
 > diff (assert the `.ir` shows the `naked` attr + `mov x0, #42` / `ret`, NO stray error
-> text). (3) Add `examples/1801-concurrency-pure-asm-x86.sx` (x86_64 body, `.build
+> text). (3) Add `examples/1802-concurrency-pure-asm-x86.sx` (x86_64 body, `.build
 > {"target":"x86_64-linux"}`, ir-only on this host — requires its `.ir`, now producible).
 > (4) Add a unit test in `src/ir/emit_llvm.test.zig` asserting the `naked` attribute is
 > present on an `abi(.pure)` function. Confirm `zig build test` green, commit. NOTE: the