fibers: rename ABI variant .pure -> .naked

"pure" universally means side-effect-free (GCC __attribute__((pure)),
FP purity, D's pure) — the opposite of a register-clobbering context
switch. The concept is "naked": no compiler-generated prologue/epilogue,
body is raw asm that emits its own ret. That is the established term
everywhere (LLVM's naked function attribute — which we literally emit —
plus Zig callconv(.naked), Rust #[naked], GCC/Clang __attribute__
((naked))). Rename the keyword + everything keyed off it so concept,
surface, field, and the emitted LLVM attribute all agree.

- ast.zig: ABI enum variant pure -> naked (+ doc).
- parser: accept abi(.naked); error text updated.
- IR Function.is_pure -> is_naked; type_resolver/decl/generic/pack/
  emit_llvm references updated; diagnostics say abi(.naked).
- examples 1800-1803 renamed *-pure-* -> *-naked-* (source + expected/
  snapshots; .ir/.exit/.stdout/.stderr are byte-identical — the emitted
  IR is unchanged, only the keyword spelling differs).
- docs (PLAN-FIBERS, CHECKPOINT-FIBERS, PLAN-POST-METATYPE, the design
  roadmap, the compiler-API checkpoint/design) updated; the naming
  rationale now records why .naked over .pure.

No semantic change — pure cosmetics. Suite green (725/0).

design/comptime-compiler-api.md

@@ -71,7 +71,7 @@ internal surface (Zig types + functions). Two defining properties:
 >   **before** `extern`/`export`. It is the unified replacement for the old
 >   `callconv(...)` (which is removed): `ABI = { default, c, zig, pure }` —
 >   `.c` (C ABI / cdecl), `.zig` (Zig-layout weld → the `compiler` library),
->   `.pure` (naked asm). `.default` = unannotated (ordinary sx convention).
+>   `.naked` (naked asm). `.default` = unannotated (ordinary sx convention).
 > - `extern <lib>` — the linkage keyword + binding source (the named library).
 
 `abi(...)` sits where `callconv(...)` went (after the return type for fns); the