feat: add remote branch diff and merge client APIs (#3686)

This PR adds some support for `diff` / `merge` in the remote client as
for local tables we stay `NotSupported` until
https://github.com/lance-format/lance/issues/7263.


This wires the two review-and-land calls against the remote REST API:
- `POST /v1/table/{id}/branches/diff`
- `POST /v1/table/{id}/branches/merge`

Rust gets typed results (`BranchDiff`, `MergeBranchResult`). Python
returns the wire JSON, same shape as the REST response.

Merge here means promoting a branch's added columns onto `main`.

### Behavior
- Remote only. Local raises `NotSupported`.
- A rejected merge is not an exception. HTTP 409 still returns `Ok` / a
dict with `status="rejected"` and blockers in `diff.mergeBlockers`.
- Unknown blocker / status codes parse as `Unknown` so a newer server
does not break older clients.
- `MergePreview` tolerates missing fields for the same reason.
- Merge requests are not retried. 409 is final and carries the body you
need.

### Example
```python

table = db.open_table("images")

table.branches.create("exp")
exp = table.branches.checkout("exp")

exp.add_columns({"tag": "cast('draft' as string)"})

diff = table.branches.diff("exp")
preview = table.branches.merge("exp", dry_run=True)
result = table.branches.merge("exp", dry_run=False)

if result["status"] == "merged":
    print("landed at", result["mainVersionAfter"])
elif result["status"] == "rejected":
    print(result["diff"]["mergeBlockers"])
```

### Testing
cargo test -p lancedb --features remote diff_branch
cargo test -p lancedb --features remote merge_branch

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/src/js/classes/Branches.md

@@ -83,6 +83,24 @@ Delete a branch.
 
 ***
 
+### diff()
+
+```ts
+diff(fromBranch): Promise<BranchDiff>
+```
+
+Compare a branch against main without modifying either branch.
+
+#### Parameters
+
+* **fromBranch**: `string`
+
+#### Returns
+
+`Promise`&lt;[`BranchDiff`](../interfaces/BranchDiff.md)&gt;
+
+***
+
 ### list()
 
 ```ts
@@ -94,3 +112,28 @@ List all branches, mapping name to branch metadata.
 #### Returns
 
 `Promise`&lt;`Record`&lt;`string`, [`BranchContents`](BranchContents.md)&gt;&gt;
+
+***
+
+### merge()
+
+```ts
+merge(fromBranch, dryRun): Promise<MergeBranchResult>
+```
+
+Merge a branch into main.
+
+Set `dryRun` to `true` to preview the merge. A rejected merge resolves
+with `status: "rejected"` instead of throwing.
+
+#### Parameters
+
+* **fromBranch**: `string`
+    Branch to merge from.
+
+* **dryRun**: `boolean` = `false`
+    When true, only preview the merge. Defaults to false.
+
+#### Returns
+
+`Promise`&lt;[`MergeBranchResult`](../interfaces/MergeBranchResult.md)&gt;

docs/src/js/globals.md

@@ -52,6 +52,11 @@
 - [AddDataOptions](interfaces/AddDataOptions.md)
 - [AddResult](interfaces/AddResult.md)
 - [AlterColumnsResult](interfaces/AlterColumnsResult.md)
+- [BranchColumnChange](interfaces/BranchColumnChange.md)
+- [BranchColumnSummary](interfaces/BranchColumnSummary.md)
+- [BranchDiff](interfaces/BranchDiff.md)
+- [BranchIndexSummary](interfaces/BranchIndexSummary.md)
+- [BranchRowCountSummary](interfaces/BranchRowCountSummary.md)
 - [ClientConfig](interfaces/ClientConfig.md)
 - [ColumnAlteration](interfaces/ColumnAlteration.md)
 - [ColumnOrdering](interfaces/ColumnOrdering.md)
@@ -86,6 +91,9 @@
 - [ListNamespacesOptions](interfaces/ListNamespacesOptions.md)
 - [ListNamespacesResponse](interfaces/ListNamespacesResponse.md)
 - [LsmWriteSpec](interfaces/LsmWriteSpec.md)
+- [MergeBlocker](interfaces/MergeBlocker.md)
+- [MergeBranchResult](interfaces/MergeBranchResult.md)
+- [MergePreview](interfaces/MergePreview.md)
 - [MergeResult](interfaces/MergeResult.md)
 - [NativeOAuthConfig](interfaces/NativeOAuthConfig.md)
 - [OAuthConfig](interfaces/OAuthConfig.md)

docs/src/js/interfaces/BranchColumnChange.md

@@ -0,0 +1,33 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / BranchColumnChange
+
+# Interface: BranchColumnChange
+
+A column whose definition differs between main and the branch.
+
+## Properties
+
+### branch
+
+```ts
+branch: BranchColumnSummary;
+```
+
+***
+
+### main
+
+```ts
+main: BranchColumnSummary;
+```
+
+***
+
+### name
+
+```ts
+name: string;
+```

docs/src/js/interfaces/BranchColumnSummary.md

@@ -0,0 +1,33 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / BranchColumnSummary
+
+# Interface: BranchColumnSummary
+
+Summary of a column in a branch diff.
+
+## Properties
+
+### dataType
+
+```ts
+dataType: string;
+```
+
+***
+
+### name
+
+```ts
+name: string;
+```
+
+***
+
+### nullable
+
+```ts
+nullable: boolean;
+```

docs/src/js/interfaces/BranchDiff.md

@@ -0,0 +1,129 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / BranchDiff
+
+# Interface: BranchDiff
+
+Read-only comparison of a branch against main.
+
+## Properties
+
+### addedColumns
+
+```ts
+addedColumns: BranchColumnSummary[];
+```
+
+***
+
+### addedIndexes
+
+```ts
+addedIndexes: BranchIndexSummary[];
+```
+
+***
+
+### baseMoved
+
+```ts
+baseMoved: boolean;
+```
+
+***
+
+### branchVersion
+
+```ts
+branchVersion: number;
+```
+
+***
+
+### changedColumns
+
+```ts
+changedColumns: BranchColumnChange[];
+```
+
+***
+
+### fromBranch
+
+```ts
+fromBranch: string;
+```
+
+***
+
+### mainVersion
+
+```ts
+mainVersion: number;
+```
+
+***
+
+### mergeBlockers
+
+```ts
+mergeBlockers: MergeBlocker[];
+```
+
+***
+
+### mergeable
+
+```ts
+mergeable: boolean;
+```
+
+***
+
+### parentVersion
+
+```ts
+parentVersion: number;
+```
+
+***
+
+### removedColumns
+
+```ts
+removedColumns: BranchColumnSummary[];
+```
+
+***
+
+### removedIndexes
+
+```ts
+removedIndexes: BranchIndexSummary[];
+```
+
+***
+
+### rowCountBranch
+
+```ts
+rowCountBranch: number;
+```
+
+***
+
+### rowCountMain
+
+```ts
+rowCountMain: number;
+```
+
+***
+
+### rowSummary
+
+```ts
+rowSummary: BranchRowCountSummary;
+```

docs/src/js/interfaces/BranchIndexSummary.md

@@ -0,0 +1,41 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / BranchIndexSummary
+
+# Interface: BranchIndexSummary
+
+Summary of an index in a branch diff.
+
+## Properties
+
+### columns
+
+```ts
+columns: string[];
+```
+
+***
+
+### indexName
+
+```ts
+indexName: string;
+```
+
+***
+
+### indexType?
+
+```ts
+optional indexType: string;
+```
+
+***
+
+### status
+
+```ts
+status: string;
+```

docs/src/js/interfaces/BranchRowCountSummary.md

@@ -0,0 +1,57 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / BranchRowCountSummary
+
+# Interface: BranchRowCountSummary
+
+Row-level comparison between main and the branch.
+
+## Properties
+
+### deltaAvailable
+
+```ts
+deltaAvailable: boolean;
+```
+
+***
+
+### inputsChanged
+
+```ts
+inputsChanged: number;
+```
+
+***
+
+### newOnBase
+
+```ts
+newOnBase: number;
+```
+
+***
+
+### newOnBranch
+
+```ts
+newOnBranch: number;
+```
+
+***
+
+### staleRecompute
+
+```ts
+staleRecompute: number;
+```
+
+***
+
+### unchanged
+
+```ts
+unchanged: number;
+```

docs/src/js/interfaces/MergeBlocker.md

@@ -0,0 +1,25 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / MergeBlocker
+
+# Interface: MergeBlocker
+
+A reason why a branch cannot currently be merged.
+
+## Properties
+
+### code
+
+```ts
+code: string;
+```
+
+***
+
+### message
+
+```ts
+message: string;
+```

docs/src/js/interfaces/MergeBranchResult.md

@@ -0,0 +1,46 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / MergeBranchResult
+
+# Interface: MergeBranchResult
+
+Result of previewing or attempting a branch merge.
+
+## Properties
+
+### diff
+
+```ts
+diff: BranchDiff;
+```
+
+***
+
+### mainVersionAfter?
+
+```ts
+optional mainVersionAfter: number;
+```
+
+***
+
+### preview
+
+```ts
+preview: MergePreview;
+```
+
+***
+
+### status
+
+```ts
+status:
+  | "unknown"
+  | "rejected"
+  | "ready"
+  | "notImplemented"
+  | "merged";
+```

docs/src/js/interfaces/MergePreview.md

@@ -0,0 +1,17 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / MergePreview
+
+# Interface: MergePreview
+
+Changes that would be, or were, promoted by a branch merge.
+
+## Properties
+
+### promotedColumns
+
+```ts
+promotedColumns: string[];
+```