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/inst.zig

@@ -640,15 +640,15 @@ pub const Function = struct {
     /// drops the leftover declaration. See current/PLAN-COMPILER-VM.md (S3).
     is_compiler_domain: bool = false,
 
-    /// True for an `abi(.pure)` function — no calling-convention
+    /// True for an `abi(.naked)` function — no calling-convention
     /// prologue/epilogue/frame, no implicit `__sx_ctx`. Its body is a single
     /// inline-asm block that reads args from ABI registers and emits its own
     /// `ret` (the context-switch primitive; design §4.6). emit_llvm lowers this
     /// via LLVM's `naked` function attribute and generates no frame setup. A
     /// `.c` epilogue would restore SP from the wrong stack across a context
-    /// switch (SP-in ≠ SP-out by design), which is why `.pure` is distinct
+    /// switch (SP-in ≠ SP-out by design), which is why `.naked` is distinct
     /// from `.c`.
-    is_pure: bool = false,
+    is_naked: bool = false,
 
     pub const Param = struct {
         name: StringId,