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).

src/ir/emit_llvm.test.zig

@@ -1325,18 +1325,18 @@ test "emit: reflectArgRepr surfaces .unresolved for an unresolvable reflection a
     try std.testing.expect(emitter.reflectArgRepr(bogus) != .bare);
 }
 
-test "emit: abi(.pure) function gets the naked attribute (no frame-pointer)" {
+test "emit: abi(.naked) function gets the naked attribute (no frame-pointer)" {
     const alloc = std.testing.allocator;
     var module = Module.init(alloc);
     defer module.deinit();
 
     var b = Builder.init(&module);
 
-    // func answer() -> i64 abi(.pure) { asm volatile { "ret" }; unreachable }
-    // The naked attribute is keyed off Function.is_pure in the declaration pass,
+    // func answer() -> i64 abi(.naked) { asm volatile { "ret" }; unreachable }
+    // The naked attribute is keyed off Function.is_naked in the declaration pass,
     // independent of the body — a minimal asm + unreachable body suffices.
     _ = b.beginFunction(str(&module, "answer"), &.{}, .i64);
-    b.currentFunc().is_pure = true;
+    b.currentFunc().is_naked = true;
     const entry = b.appendBlock(str(&module, "entry"), &.{});
     b.switchToBlock(entry);