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/lower/pack.zig

@@ -949,7 +949,7 @@ pub fn monomorphizePackFn(
     const name_id = self.module.types.internString(owned_name);
     _ = self.builder.beginFunction(name_id, params.items, ret_ty);
     self.builder.currentFunc().has_implicit_ctx = wants_ctx;
-    self.builder.currentFunc().is_pure = (fd.abi == .pure);
+    self.builder.currentFunc().is_naked = (fd.abi == .naked);
 
     const entry_name = self.module.types.internString("entry");
     const entry = self.builder.appendBlock(entry_name, &.{});
@@ -1039,10 +1039,10 @@ pub fn monomorphizePackFn(
     defer self.setCurrentSourceFile(saved_source);
     if (fd.body.source_file) |src| self.setCurrentSourceFile(src);
 
-    if (self.builder.currentFunc().is_pure) {
-        // `abi(.pure)`: asm-only body that rets itself — no sx value return.
+    if (self.builder.currentFunc().is_naked) {
+        // `abi(.naked)`: asm-only body that rets itself — no sx value return.
         // Lower statements + cap with `unreachable` (mirrors the decl path).
-        // emit_llvm bails on `is_pure` until B1.0b implements `naked` emission.
+        // emit_llvm bails on `is_naked` until B1.0b implements `naked` emission.
         self.lowerBlock(fd.body);
         if (!self.currentBlockHasTerminator()) self.builder.emitUnreachable();
     } else if (ret_ty != .void) {