fibers B1.3a-1: stackful context switch (naked swap_context + fiber bootstrap)

The first piece of the B1.3 fiber runtime — the stackful context switch, pure
sx over abi(.naked). swap_context(from, to) saves the callee-saved registers +
SP/LR into *from and loads them from *to, then rets onto to's stack (SP-in !=
SP-out by design — why it must be .naked). Fibers are bootstrapped by hand: the
saved context starts with SP = top of an alloc_bytes stack, LR = a global-asm
trampoline (mov x0, x19; bl _fib_body, reaching the sx body via export), and
x19 = the *Fiber.

Locked by examples/1807-concurrency-fiber-context-switch.sx (aarch64-pinned):
- 2-fiber ping-pong (A <-> B, 3 rounds each): rounds: 6, and a per-fiber stack
  canary held live across every suspend survives (canary fails: 0);
- a 64-frame deep recursive chain suspended at the bottom and resumed, verifying
  every frame's stack-local on the unwind (frames verified: 64, depth fails: 0).

Scope (honest): exercises register/stack preservation INDIRECTLY (compiler-
allocated live values + the canary). The EXPLICIT every-callee-saved GP
(x19-x28) + FP (d8-d15) sentinel scribble — the full design-section-10.7 gate —
is B1.3a-2, still owed. x86_64 sibling + mmap guard-page stacks are B1.3b.

Suite green 733/0. Runs under JIT, ir-only on a non-arm host.

examples/expected/1807-concurrency-fiber-context-switch.build

@@ -0,0 +1 @@
+{ "target": "macos" }

examples/expected/1807-concurrency-fiber-context-switch.exit

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+0

examples/expected/1807-concurrency-fiber-context-switch.stderr

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+

examples/expected/1807-concurrency-fiber-context-switch.stdout

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+rounds: 6
+canary fails: 0
+frames verified: 64
+depth fails: 0