Problem: integration keys (`fugitive`, `neogit`, `gitsigns`, `committia`,
`telescope`) live at the top level of `vim.g.diffs`, cluttering the
config alongside unrelated options like `highlights` and `conflict`.
Solution: move them under `vim.g.diffs.integrations.*`. Old top-level
keys are migrated automatically with a `vim.deprecate` warning targeting
v0.3.2. `compute_filetypes` and `plugin/diffs.lua` fall back to legacy
keys for pre-`init()` callers.