Silent merge corruption is the worst failure a version-control system can have — it produces a tree that looks right — and merge3 had no fuzz target. This adds one, driving the merger with generated base/current/other triples and asserting the properties that must hold for any input.

Oracle 4 (no invented lines) needs a carve-out, because the merger is supposed to fabricate lines inside a conflict envelope. That carve-out is only sound if the recorded spans really are envelopes, so expectNoInventedLines validates the claim before subtracting anything: each span must be in-bounds, open at a line boundary with conflict_sentinel, close with conflict_terminator, start at or after the previous span's end, and carry no second terminator inside it.

The ordering and interior-terminator checks are load-bearing, not belt-and- braces. inAnySpan matches on overlap, so an over-long span exempts every output line up to its end. End-pinning alone admits a single span stretched from the first envelope's start to the last one's end — it opens and closes correctly and overlaps nothing — which would exempt the clean lines between two envelopes from the oracle entirely. closes_once catches that case on the first envelope's terminator. merge3 asserts ordering itself, but only under runtime_safety, which a ReleaseFast sweep — the build a fuzz run uses — erases; this loop holds in every optimize mode.

merge3.zig gains one export: conflict_terminator, the closing counterpart of the existing conflict_sentinel, with a comptime tripwire pinning its shape. Additive only, no behavior change — it lets the oracle pin both envelope ends against the module that produces them instead of re-typing the literal, so the two cannot drift apart.

Battery green in the worktree: zig fmt --check ., zig build, zig build test, zig build e2e, zig build dst (6 seeds x 1000 steps). The new target runs clean at seed 1 over 2000 rounds; no bug was found, so no regression seed is pinned. closed check-in: 21ca364fb3 user: dmestas tags: release-0.1