test: add device exclusion and defseq preservation tests

[malpern]

Context

Two config generation features have minimal test coverage:

1. Device exclusion — the (defcfg ... macos-dev-names-include ...) block that targets specific keyboards. No tests verify this renders correctly from the device list.

2. Sequence/defseq preservation — the generation pipeline loads preserved sequences and chord groups from disk (loadPreservedSequences(), loadPreservedChordGroups()) and injects them into the generated config. This round-trip path has no dedicated tests.

Gaps to address

• Device exclusion block rendering from a list of device hashes
• Preserved defseq blocks survive config regeneration
• Preserved defchords blocks survive config regeneration
• Edge case: empty device list, empty sequences

Priority

Medium — device exclusion bugs affect multi-keyboard users; defseq bugs lose user sequences on config save.

See docs/TEST_COVERAGE_REPORT.md for full audit.