Feature: SQL Window Functions → MongoDB `$setWindowFields` (ROW_NUMBER, RANK, DENSE_RANK, RUNNING SUM/AVG)

## **Summary**
Add translation support for common SQL window functions into MongoDB’s `$setWindowFields` (MongoDB ≥ 5.0). Cover `ROW_NUMBER()`, `RANK()`, `DENSE_RANK()`, `NTILE(n)` (best-effort), running/tumbling `SUM/AVG/MIN/MAX/COUNT`, and partitioned order-aware windows. Provide graceful fallback/errors for unsupported server versions.

## Motivation

* Window functions are standard in analytics SQL; many migrations stall when they hit `OVER (PARTITION BY … ORDER BY …)`.
* MongoDB’s `$setWindowFields` is the canonical equivalent; supporting it unlocks powerful reporting in-place without ETL.

## Scope (v1)

Supported SQL patterns:

* `ROW_NUMBER() OVER (PARTITION BY … ORDER BY …)`
* `RANK()` / `DENSE_RANK()` over partition+order
* Running aggregates: `SUM/AVG/MIN/MAX/COUNT` with `ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW`
* Basic frame variants: `CURRENT ROW`, `UNBOUNDED PRECEDING`; (optional) `N PRECEDING` where feasible
* `NTILE(n)` (emit bucket index via computed percentile approximation)

Out of scope (follow-ups):

* Window frames with `FOLLOWING`, `RANGE` (value-based) frames, and complex expressions
* `LAG/LEAD` (separate issue)

## API

```python
from sql_mongo_converter import sql_to_pipeline

pipeline = sql_to_pipeline("""
SELECT
u.country,
u.user_id,
ROW_NUMBER() OVER (PARTITION BY u.country ORDER BY u.created_at) AS rn,
RANK() OVER (PARTITION BY u.country ORDER BY u.total_spend DESC) AS rnk,
SUM(u.total_spend) OVER (
PARTITION BY u.country
ORDER BY u.created_at
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
) AS running_spend
FROM users u
WHERE u.status = 'ACTIVE';
""", options={
"mongoServerVersion": "6.0", # enable $setWindowFields
"emitCompatibilityNotes": True
})
```

### Example Output (abridged)

```json
{
"collection": "users",
"pipeline": [
{"$match": {"status": "ACTIVE"}},
{"$setWindowFields": {
"partitionBy": "$country",
"sortBy": {"created_at": 1},
"output": {
"rn": {"$documentNumber": {}}, // ROW_NUMBER
"running_spend": {
"$sum": "$total_spend",
"window": {"documents": ["unbounded","current"]}
}
}
}},
{"$setWindowFields": {
"partitionBy": "$country",
"sortBy": {"total_spend": -1},
"output": {
"rnk": {"$rank": {}} // RANK (DENSE_RANK → $denseRank)
}
}},
{"$project": {"country": 1, "user_id": 1, "rn": 1, "rnk": 1, "running_spend": 1, "_id": 0}}
]
}
```

> Note: multiple `$setWindowFields` stages may be emitted to accommodate different `ORDER BY` specs per window.

## Translation Rules

* **PARTITION BY** → `partitionBy: `
* **ORDER BY** → `sortBy: { … }`
* **ROW_NUMBER** → `"$documentNumber": {}`
* **RANK** → `"$rank": {}`, **DENSE_RANK** → `"$denseRank": {}`
* **Running aggregates** → accumulator + `window.documents: ["unbounded","current"]`
* **`N PRECEDING`** (best-effort) → `window.documents: [-N, "current"]` (warn when not supported)
* **NTILE(n)** (approx)**:** compute bucket index using `$rank`/`$denseRank` + derived math; document approximation caveats.

## Dialect & Validation

* Gate feature behind `options.mongoServerVersion >= 5.0`; otherwise raise a clear, actionable error with a suggestion to precompute or upgrade.
* Validate that each window has a single `ORDER BY` and compatible frame; emit warnings for unsupported frames.

## Tests

* **Row number by partition** (stable order)
* **Rank vs dense_rank** with ties
* **Running sum/avg** by time order
* **Mixed windows** (different ORDER BYs) on same query
* **Version guard** (4.4 → friendly error)
* **NTILE(n)** sanity checks on uniform partitions

## README / Docs

* New section: **“Window Functions → $setWindowFields”**
* Matrix table (SQL function → Mongo expression)
* Two full copy-paste examples + caveats (frames, server version)
* Performance note: ensure supporting indexes on `partitionBy` and `sortBy` keys

## Acceptance Criteria

* [ ] `sql_to_pipeline()` translates listed window functions into valid `$setWindowFields` stages.
* [ ] Multiple windows with different `ORDER BY` are supported (multiple stages emitted).
* [ ] Version gating & helpful error/warning messages implemented.
* [ ] Comprehensive unit tests + README examples added.
* [ ] Backward compatibility preserved for existing APIs.