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).

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

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

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

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

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

@@ -0,0 +1 @@
+