[Feature Request]: Enhance data branch diff with fallback, delta output, and summary statistics

## Is there an existing issue for the same feature request?

- [x] I have checked the existing issues.

## Is your feature request related to a problem?

Currently `data branch diff` has limitations that make it less flexible than Git's diff:

1. **Only works with branched tables**: Requires LCA (Lowest Common Ancestor) relationship, cannot compare arbitrary tables
2. **Limited output modes**: Only supports full output and `output count`, lacks incremental change tracking
3. **No detailed statistics**: `output count` only returns total rows, not breakdown of changes

## Describe the feature you'd like

### 1. Fallback to EXCEPT for non-branched tables

**Current behavior:**
```sql
-- Only works if t2 was created from t1
data branch create table t2 from t1{snapshot="sp1"};
data branch diff t2 against t1; -- ✅ Works (has LCA)

-- Fails if no branch relationship
data branch diff t3 against t4; -- ❌ Error: no LCA
```

**Proposed behavior:**
```sql
-- Automatically fallback to EXCEPT-like behavior
data branch diff t3 against t4; -- ✅ Works (uses set difference)
```

### 2. Delta output mode

**Proposed syntax:**
```sql
data branch diff t1 against t2 output delta;
```

**Expected result:**
```
operation | a | b
----------|---|---
+ | 3 | 3 -- Added in t1
- | 4 | 4 -- Deleted from t1 (exists in t2)
```

Similar to Git's `+`/`-` notation for added/removed lines.

### 3. Summary statistics output

**Proposed syntax:**
```sql
data branch diff t1 against t2 output summary;
```

**Expected result:**
```
metric | count
----------|------
added | 10
deleted | 5
modified | 3
unchanged | 100
```

Provides quick overview without scanning all rows.

## Describe implementation you've considered

### Fallback mechanism
- Check if tables have LCA relationship
- If no LCA: use EXCEPT-based comparison
- Maintain same output format for consistency

### Delta output
- Add `operation` column: `+` (added), `-` (deleted), `~` (modified)
- Use primary key to match rows for modification detection
- If no PK: treat as add/delete only

### Summary statistics
- Leverage existing diff computation
- Aggregate results into statistics
- Minimal performance overhead (single pass)

## Documentation, Adoption, Use Case, Migration Strategy

### Use Cases

1. **Data validation**: Compare staging vs production tables without branch relationship
2. **Change tracking**: Quickly see what changed between snapshots
3. **Monitoring**: Track data drift with summary statistics
4. **Debugging**: Identify specific rows added/deleted with delta output

### Adoption

- Backward compatible: existing queries work unchanged
- New output modes are opt-in via `output` clause
- Fallback behavior is transparent to users