[Repo Assist] Fix Frame.writeParquet writing empty files (no row data)

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🤖 This is an automated PR from Repo Assist.

Closes #712

Root Cause

Frame.writeParquet and Frame.writeParquetStream were silently writing empty files — only the column schema was persisted, but all row data was discarded.

The bug is a one-character fix in src/Deedle.Parquet/Parquet.fs:

// Before (broken) — row group never committed:
let rg = writer.CreateRowGroup()

// After (fixed) — Dispose() commits the row group to the stream:
use rg = writer.CreateRowGroup()
```

The `IParquetRowGroupWriter` returned by `ParquetWriter.CreateRowGroup()` implements `IDisposable`. Disposing it is what causes Parquet.Net to flush and commit the row group's data to the underlying stream. Using `let` instead of `use` meant `Dispose()` was never called, so the writer's internal buffers were discarded when GC collected them — or in practice just never flushed. The schema header was written by the `ParquetWriter` itself, which is why column names appeared but no rows.

The same bug affected `writeParquetStream`.

## Changes

| File | Change |
|---|---|
| `src/Deedle.Parquet/Parquet.fs` | `let rg` → `use rg` in `writeParquet` and `writeParquetStream` |
| `tests/Deedle.Parquet.Tests/data/weather.parquet` | New: meteorological sample (8 rows × 7 cols, PyArrow-generated) |
| `tests/Deedle.Parquet.Tests/data/trades.parquet` | New: financial trades sample (20 rows × 7 cols, PyArrow-generated) |
| `tests/Deedle.Parquet.Tests/data/sensors.parquet` | New: IoT sensor log (10 rows × 7 cols, PyArrow-generated) |
| `tests/Deedle.Parquet.Tests/Tests.fs` | 20 new tests for the three sample files |
| `RELEASE_NOTES.md` | Bug-fix entry under new 6.0.2 section |

### New sample files

The three new `.parquet` files were generated with PyArrow 23, providing cross-tool coverage (Deedle reads files written by a different Parquet library):

- **`weather.parquet`** — `string`, `DateTime`, `float64`, `int32`, `float32`, `float64` (with nulls), `bool`
- **`trades.parquet`** — `int32`, `string`, `float64`, `int64`, `int16`, `DateTime`, `bool`
- **`sensors.parquet`** — `int32`, `float32` (nulls), `float64` (nulls), `int32` (nulls), `int64` (nulls), `string` (nulls), `bool` (nulls) — exercises every nullable path

## Test Status

```
Passed! - Failed: 0, Passed: 55, Skipped: 0, Total: 55

All 55 tests pass (35 pre-existing + 20 new).

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