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/execution-evolution-roadmap.md

@@ -396,10 +396,10 @@ grounding) are explicit steps, not buried.
      capability:** name minted results by the instantiation's mangled name + input
      validation.
 4. **`abi(.naked)`** — *correction:* `CallConv` was renamed `ABI` and **already carries
-   `.pure`** (ast.zig:142, "pure/naked, no prologue/epilogue") during the compiler-API
-   stream — so this is NOT "extend the enum." `.pure` is an **inert label today**:
+   `.naked`** (ast.zig:142, "naked, no prologue/epilogue") during the compiler-API
+   stream — so this is NOT "extend the enum." `.naked` is an **inert label today**:
    `type_resolver.zig:237` maps it to `.default` CC and emit_llvm emits **no** naked
-   attribute. The net-new work is making `.pure` actually emit LLVM `naked` + skip
+   attribute. The net-new work is making `.naked` actually emit LLVM `naked` + skip
    prologue/epilogue lowering. Gates A2.
 5. **Per-fiber `context` root + push-stack storage** — *correction:* `context` is
    **already an implicit `*Context` parameter** (comptime_vm.zig:392, lower.zig:257