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/token.zig

@@ -42,7 +42,7 @@ pub const Tag = enum {
     kw_impl, // impl
     kw_Self, // Self (in protocol declarations)
     kw_inline, // inline (compile-time if/for/while)
-    kw_abi, // abi (ABI / calling-convention annotation: abi(.c)/abi(.zig)/abi(.pure))
+    kw_abi, // abi (ABI / calling-convention annotation: abi(.c)/abi(.zig)/abi(.naked))
     kw_extern, // extern (import: external linkage, C ABI, no body)
     kw_export, // export (define + expose: external linkage, C ABI)
     kw_asm, // asm (inline assembly expression / global asm decl)