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feat(FN-4072): add diff volume gate to merger

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Added a diff volume gate to the merger that blocks or warns on large diffs before merge completes. The feature includes a new `merger-diff-volume-gate` module wired into the merger, configurable via project settings, with tests covering all gate paths. Also updated docs and the changeset for the `@r

Fusion-Task-Id: FN-4072
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Fusion
2026-05-12 18:23:13 -07:00
committed by gsxdsm
parent fbd441e31c
commit 8b037e4c7e
8 changed files with 660 additions and 6 deletions
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docs/settings-reference.md
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@@ -184,6 +184,9 @@ Defaults from `DEFAULT_PROJECT_SETTINGS`; key scope from `PROJECT_SETTINGS_KEYS`
| `mergeStrategy` | `"direct" \| "pull-request"` | `"direct"` | Completion mode (local direct merge vs PR-first). |
| `directMergeCommitStrategy` | `"auto" \| "always-squash" \| "always-rebase"` | `"auto"` | Direct-merge commit routing mode. `auto` keeps the legacy squash path for branches with zero or one substantive commit, but switches multi-substantive direct merges to a history-preserving rebase-and-merge/cherry-pick path so commit boundaries, subjects, and `Fusion-Task-Id` trailers survive on `main`. `always-squash` forces the legacy squash path; `always-rebase` always preserves per-commit history. Only applies when `mergeStrategy="direct"`. |
| `mergeConflictStrategy` | `"smart-prefer-main" \| "smart-prefer-branch" \| "ai-only" \| "abort"` | `"smart-prefer-main"` | Controls the merger's conflict-resolution cascade. `smart-prefer-main` fast-forwards local main from `origin` when possible, then tries AI resolution, then auto-resolve heuristics, then a final `-X ours` fallback that prefers main unless the overlap guard below says otherwise. `smart-prefer-branch` uses the same cascade but ends with `-X theirs` so the task branch wins. `ai-only` never silently picks a side, and `abort` stops after the first AI attempt. Legacy `smart` / `prefer-main` values are normalized automatically. |
| `mergeDiffVolumeMinLines` | `number` | `20` | Minimum branch-net line volume before Fusion compares a file's staged squash delta against the branch's net delta. Applied at merge time and clamped to `>= 1`. |
| `mergeDiffVolumeThreshold` | `number` | `0.2` | Minimum staged-to-branch-net ratio allowed for a non-allowlisted file during auto-resolved squash finalization. Applied at merge time and clamped to `0..1`. |
| `mergeDiffVolumeAllowlist` | `string[]` | `[]` | Additional glob patterns skipped by the pre-commit diff-volume gate, beyond the built-in generated-file and lockfile allowlists. |
| `mergeStrategyOverlapBehavior` | `"flip-to-prefer-branch" \| "warn-only" \| "ignore"` | `"flip-to-prefer-branch"` | Safety control for `mergeConflictStrategy="smart-prefer-main"`. Before the Attempt 3 `-X ours` fallback, Fusion checks whether the task branch and recent `main` history overlap on the same files (30-commit lookback, matching the squash audit heuristics). `flip-to-prefer-branch` makes overlapping files prefer the task branch so hardening is not silently discarded (the FN-3936 class of regression). `warn-only` logs the overlap but keeps the legacy main-wins fallback. `ignore` disables the overlap guard and preserves legacy behavior exactly. |
### Per-task direct-merge override