Add indexed menu snapshot composition foundation

[stevenboyd78]

Summary

• add SDSScanner.open_indexed_menu_snapshot() to compose the existing indexed MNU acknowledgement and subsequent MSI retrieval under one host-side command lock
• apply one total timeout budget across lock acquisition, MNU, and MSI while preflighting MSI transport support before opening the menu
• preserve the existing lossless MsiResponse and existing MSI transport restrictions without introducing new wire forms or menu lifecycle ownership
• add regression coverage for exact serial composition, fail-closed unsupported transports, total timeout budgeting, and lock serialization
• harden the pre-existing PSI renewal concurrency test after repeated Python 3.12 CI timing failures by increasing only its synthetic renewal timeout from 0.1 to 0.5 seconds; production behavior is unchanged
• update Milestone 24.8 roadmap/research documentation with the seventh-slice scope and boundaries

Validation

• Ruff: passed
• MyPy: passed across 137 source files
• pytest: 3628 passed
• affected PSI renewal test: 25 consecutive targeted passes after hardening
• tests/test_radio.py: 62 passed
• documentation checks: passed for 47 Markdown files
• package build: sds200-0.20.2 wheel and sdist built successfully
• Twine check: passed
• original feature head 2d4264ebbddbaf3ea0cbf82007d58f8f112ea314: push CI attempt 2 passed all five jobs after an isolated Python 3.12 rerun
• subsequent PR CI reproduced the same pre-existing PSI renewal timing failure on Python 3.12; correction commit be7f83f238ff20fb96e5f19087a6ed52bdaeed9d changes only that test timeout from 0.1 to 0.5 seconds
• corrected head requires fully green exact-head push and PR CI before merge

Scope boundaries

This slice is host-side serialization only. It does not add MSV/MSB, unindexed MNU, automatic menu exit/back behavior, scanner-side transaction/atomicity claims, fallback MSI support, renderer behavior, firmware/model applicability claims, or physical scanner validation.