Problem: regressions #119 and #120 exposed gaps in coverage of the
decoration provider cache pipeline, the end-to-end highlight
pipeline, and Neogit-specific behavior.
Solution: add three spec files covering:
- decoration_provider_spec: cache population, fingerprint guard,
pending_clear semantics, BufWipeout cleanup via _test table
- integration_spec: full buffer to attach to extmarks pipeline
- neogit_integration_spec: neogit_disable_hunk_highlight and
filetype attachment
Closes#122
