Panic on TIFFs that omit the SamplesPerPixel tag (spec default is 1)

## Summary

Parsing a TIFF that omits the `SamplesPerPixel` tag (277) panics with `samples_per_pixel not found` instead of applying the spec default.

The TIFF 6.0 specification defines a **default value of 1** for `SamplesPerPixel` ([tag 277 reference](https://download.osgeo.org/libtiff/doc/TIFF6.pdf)). Many single-channel grayscale TIFFs omit the tag entirely — notably the PerkinElmer/Phenix microscopy images in the [Cell Painting Gallery](https://registry.opendata.aws/cellpainting-gallery/) open dataset.

## Where

`src/ifd.rs`:

```rust
let samples_per_pixel = samples_per_pixel.expect("samples_per_pixel not found");
```

Because this is an `.expect()` inside the async IFD parse, it surfaces (via `virtual_tiff`/`pyo3-async-runtimes`) as an uncatchable Rust panic:

```
thread 'tokio-rt-worker' panicked at .../async-tiff/src/ifd.rs:375:51:
samples_per_pixel not found
```

## Reproduce

Any TIFF lacking tag 277 triggers it. A public example (single-channel 16-bit LZW grayscale, no `SamplesPerPixel` tag):

```
s3://cellpainting-gallery/cpg0000-jump-pilot/source_4/images/2020_11_04_CPJUMP1/images/BR00116991__2020-11-05T19_51_35-Measurement1/Images/r01c01f01p01-ch1sk1fk1fl1.tiff
```

(publicly readable with `--no-sign-request`).

Confirmed absent via `tifffile`:

```python
import tifffile
p = tifffile.TiffFile("r01c01f01p01-ch1sk1fk1fl1.tiff").pages[0]
assert 277 not in [t.code for t in p.tags] # SamplesPerPixel omitted
```

## Suggested fix

Fall back to the spec default of 1:

```rust
let samples_per_pixel = samples_per_pixel.unwrap_or(1);
```

The existing `PlanarConfiguration` logic just below already handles the `samples_per_pixel == 1` case, so this is a self-contained change. Happy to open a PR (with a minimal regression fixture).

Start in src/ifd.rs at the SamplesPerPixel expect() shown in the issue, then read the nearby PlanarConfiguration logic. Reproduce with a TIFF that omits tag 277, such as the referenced Cell Painting Gallery image, and add a minimal regression fixture. Done means the file parses without a panic and uses the TIFF default of one sample per pixel.

Written by the indexing model from the issue text.