fix(ios): report async pool exhaustion as busy

Summary

• add the public BUSY native bridge error code
• report all iOS async reply-pool exhaustion as retryable capacity contention
• report a second image, contact, or file picker request as BUSY without displacing the first request
• remove stale comments that described INVALID_PARAMETER as a temporary stand-in

Closes #242.

The broader stale CraftErrorCode declaration vocabulary is tracked separately in #266.

Verification

• zig build test:ios
• bun run test:sdk (566 pass)
• bun run typecheck
• bun run verify:packages (36 checks)
• bunx --bun pickier . (0 errors, existing 36 warnings)
• zig fmt --check for all touched Zig files

Regression tests were also mutation-checked: changing the wire code, pool-full result, or already-presented result back to their previous values each produces a targeted failing test.

bun run test reaches an unrelated compile failure in the sibling zig-js checkout under the locally installed Zig 0.17 development build (ArrayList API/layout incompatibility). The focused iOS suite is green.