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.zig

@@ -408,9 +408,9 @@ pub const LLVMEmitter = struct {
             // its only references are in comptime code, so DCE drops the leftover
             // declaration. See current/PLAN-COMPILER-VM.md (S3).
             if (func.is_compiler_domain) continue;
-            // `abi(.pure)` functions emit normally — the `naked` attribute (set
+            // `abi(.naked)` functions emit normally — the `naked` attribute (set
             // in the declaration pass) makes the backend emit the body (inline
-            // asm + its own `ret`) with no prologue/epilogue. See Function.is_pure.
+            // asm + its own `ret`) with no prologue/epilogue. See Function.is_naked.
             self.emitFunction(&func, @intCast(i));
         }
 
@@ -1327,13 +1327,13 @@ pub const LLVMEmitter = struct {
         // Add frame-pointer and nounwind attributes for correct ARM64 codegen
         {
             const func_idx_attr: c.LLVMAttributeIndex = @bitCast(@as(i32, -1));
-            if (func.is_pure) {
-                // `abi(.pure)`: emit via LLVM's `naked` attribute — the backend
+            if (func.is_naked) {
+                // `abi(.naked)`: emit via LLVM's `naked` attribute — the backend
                 // emits the body verbatim (our inline asm + its own `ret`) with
                 // NO prologue/epilogue/frame. Do NOT request `frame-pointer`
                 // (incompatible with a frameless function). `noinline` keeps the
                 // asm body out of a framed caller; `nounwind` — naked asm never
-                // unwinds. See Function.is_pure / current/PLAN-FIBERS.md.
+                // unwinds. See Function.is_naked / current/PLAN-FIBERS.md.
                 const naked_id = c.LLVMGetEnumAttributeKindForName("naked", 5);
                 c.LLVMAddAttributeAtIndex(llvm_func, func_idx_attr, c.LLVMCreateEnumAttribute(self.context, naked_id, 0));
                 const noinline_id = c.LLVMGetEnumAttributeKindForName("noinline", 8);