[superlzyguy]

doltMerge() currently returns fast_forward: 0 even when HEAD advances to exactly the incoming commit. Capture the native merge result and report 1 when it equals the resolved merge input. Resolve the source once and merge that immutable hash, so concurrent branch movement cannot change classification; preserve the caller's branch label in the merge commit message.

Adds six tests: true fast-forward, three-way merge, already-up-to-date, fast-forward to a merge commit, quoted branch label, and missing source without a HEAD change.

Provenance:

• Applies the recorded patch unchanged; SHA256 ad3964c77fd49e153616cec9fd7271060f490e583ee46feec69996483df67964.
• Recorded patch and historical validation: 135 wrapper tests, direct three-outcome probe, engine merge checks, and deterministic branch-movement probe on the earlier source build.
• Installed-package diagnostic: node scripts/native-followup-probe.mjs, official 0.50.9, Linux x64, Node 24.21.0, rechecked October 19: headEqualsIncoming=true, engine fastForward=false. Diagnostic exits 1 as intended (it also reproduces an independent native reset defect).

This upstream submission does not change the consumer's official 0.50.9 dependency or publish a fork package.

Fresh Linux x64 validation on upstream base 3c1824d5bf24f1b6c2e681d366fc02a395665809 using the official 0.51.0 release amalgamation, Node 24.21.0 and Bun 1.4.2:

• npm install --ignore-scripts --package-lock=false, node scripts/download.js, node scripts/build-amalgamation.js, node-gyp rebuild succeeded.
• bun test test/: 135 pass, 0 fail (6 files, 228 assertions). The checkout contains no prebuilt addon, so tests load build/Release/doltlite.node.
• Recompiled only the original wrapper source with node-gyp build and reran the same tests: 133 pass, exactly two failures (the two true fast-forward cases). Restored the patch afterward.