style(rust/signed-doc): simplified iterator call chain

[no30bit]

Description

Logic is maintained while code condensed and seems simpler.
Couldn't make a classic gh suggestion, because the code spans deleted lines.

Related Pull Requests

#317

Please confirm the following checks

• My code follows the style guidelines of this project
• [x I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 320/320}$ | ${\color{red}Fail: 0/320}$ |