libviprs 0.3.1

# Migrating from libviprs 0.2.0 to 0.3.0

0.3.0 collapses every pyramid entry point into a single `EngineBuilder` and
flips `FsSink::new` to a 2-arg constructor plus a `with_format` builder. This
guide covers the call sites you are most likely to update.

## `FsSink`

The third format argument is gone. Set the format via the builder; default is
`TileFormat::Png`.

```rust
// 0.2.0
let sink = FsSink::new("out", plan.clone(), TileFormat::Png);

// 0.3.0
let sink = FsSink::new("out", plan.clone()).with_format(TileFormat::Png);
```

`FsSink::new_with_format(...)` still compiles as a deprecated alias of
`FsSink::new(...).with_format(...)`.

## Free `generate_pyramid_*` functions → `EngineBuilder`

All five free functions plus `generate_pyramid_resumable` are removed. Replace
each call with `EngineBuilder::new(source, plan, sink)` and pick the engine
via `with_engine(EngineKind::...)` (omit it for `Auto`).

### Monolithic (in-memory)

```rust
// 0.2.0
let result = generate_pyramid(&raster, &plan, &sink, &config)?;

// 0.3.0
let result = EngineBuilder::new(&raster, plan, sink)
    .with_config(config)
    .run()?;
```

### Observed monolithic

```rust
// 0.2.0
let result = generate_pyramid_observed(&raster, &plan, &sink, &config, &observer)?;

// 0.3.0
let result = EngineBuilder::new(&raster, plan, sink)
    .with_config(config)
    .with_observer(observer)
    .run()?;
```

### Streaming

```rust
// 0.2.0
let result = generate_pyramid_streaming(&strip_src, &plan, &sink, &cfg, &observer)?;

// 0.3.0
let result = EngineBuilder::new(strip_src, plan, sink)
    .with_engine(EngineKind::Streaming)
    .with_memory_budget(cfg.memory_budget_bytes)
    .with_budget_policy(cfg.budget_policy)
    .with_observer(observer)
    .run()?;
```

### MapReduce (and the `_auto` variant)

```rust
// 0.2.0
let result = generate_pyramid_mapreduce(&strip_src, &plan, &sink, &cfg, &observer)?;
// or
let result = generate_pyramid_mapreduce_auto(&strip_src, &plan, &sink, &cfg, &observer)?;

// 0.3.0
let result = EngineBuilder::new(strip_src, plan, sink)
    .with_engine(EngineKind::MapReduce)
    .with_observer(observer)
    .run()?;
```

`EngineKind::Auto` (the default) picks Monolithic, Streaming, or MapReduce
based on the source kind and memory budget.

### Resumable

`generate_pyramid_resumable` is absorbed into `EngineBuilder` and works for
every engine, not just the monolithic path.

```rust
// 0.2.0
let result = generate_pyramid_resumable(
    &raster, &plan, &sink, &config, &observer, checkpoint_root,
)?;

// 0.3.0
let result = EngineBuilder::new(&raster, plan, sink)
    .with_config(config)
    .with_observer(observer)
    .with_resume(
        ResumePolicy::resume()
            .with_checkpoint_root(checkpoint_root)
            .with_checkpoint_every(64),
    )
    .run()?;
```

`ResumePolicy::overwrite()`, `::resume()`, and `::verify()` anchor the mode;
`with_checkpoint_every` and `with_checkpoint_root` tune persistence.
`Default` is `Overwrite`.

## Observers / events

`EngineEvent` now covers the full pipeline lifecycle:

- `SourceLoadStarted { source_description }`
- `SourceLoaded { width, height, format: PixelFormat, size_bytes }`
- `PlanCreated { levels, total_tiles, canvas_width, canvas_height }`
- `LevelStarted { level, width, height, tile_count }`
- `TileCompleted { coord }`
- `LevelCompleted { level, tiles_produced }`
- `StripRendered { strip_index, total_strips }` (Streaming)
- `BatchStarted { batch_index, strips_in_batch, total_batches }` (MapReduce)
- `BatchCompleted { batch_index, tiles_produced }` (MapReduce)
- `Finished { total_tiles, levels }`
- `PipelineComplete`

`EngineBuilder::with_observer(impl EngineObserver + 'static)` and
`with_observer_arc(Arc<dyn EngineObserver>)` feed every engine — Monolithic,
Streaming, and MapReduce — so a single observer implementation works against
all of them. `PixelFormat` is now public and re-exported at the crate root.

## Cargo features

| Feature | Default | Purpose |
|---|---|---|
| `pdfium` | off | Vector PDF rendering, `PdfiumStripSource`, `render_page_pdfium*` |
| `pdfium-static` | off | New in 0.3.0 — pulls in `pdfium` plus `pdfium-render/static` for static linking of libpdfium |
| `s3` | off | Gates `ObjectStoreSink` against a user-injected `ObjectStore` |
| `tracing` | off | Structured spans/events |
| `packfile` | off | `PackfileSink` (write tiles into a tar/zip), now with `PackfileSinkBuilder` |

`default = []` — no features enabled by default. MSRV is 1.85, edition 2024.