Add THR export and segment subsampling pipeline stage (#166)

## Summary

- Add THR (Theta-Rho) serializer for polar sand tables (Sisyphus, Oasis,
Dune Weaver) with `atan2(x,y)` convention, continuous theta unwinding,
rho normalization to [0,1], metadata comment headers, and 5-decimal
precision matching Sandify
- Add segment subsampling as pipeline stage 9 (after join) to subdivide
long Cartesian line segments before polar conversion, preventing
unexpected arcs in THR output
- Enable THR checkbox in the export panel with download wiring

## Details

### New files
- **`crates/mujou-export/src/thr.rs`** — THR serializer with ~20 unit
tests + 1 end-to-end test
- **`crates/mujou-pipeline/src/subsample.rs`** — `subsample()` function
with 18 unit tests

### Pipeline changes
- `STAGE_COUNT` bumped to 10 with new `Subsampled` typed stage
- `PipelineConfig.subsample_max_length` (default 2.0 px)
- `StagedResult.subsampled` field; `final_polyline()` returns subsampled
output
- `StageMetrics::Subsample` variant in diagnostics
- Cache resume support for the new stage

### UI changes
- THR checkbox enabled in export panel with Blob download
- `StageId` mapping updated for new pipeline index

### Tests
- ~40 new unit tests across `thr.rs` and `subsample.rs`
- 2 cherry blossoms integration tests
(`cherry_blossoms_pipeline_to_thr`,
`cherry_blossoms_pipeline_to_thr_with_mask`)
- All 380 tests pass, zero clippy warnings

### Docs
- THR format spec added to `formats.md`
- Implementation checklist updated (Phases 0–5 checked off)

Author: altendky

crates/mujou-export/src/lib.rs

@@ -1,8 +1,10 @@
 //! mujou-export: Pure format serializers (sans-IO)
 //!
-//! Converts polylines into output formats. Currently supports SVG.
-//! Future formats: THR, G-code, DXF, PNG.
+//! Converts polylines into output formats. Currently supports SVG and THR.
+//! Future formats: G-code, DXF, PNG.
 
 pub mod svg;
+pub mod thr;
 
 pub use svg::{SvgMetadata, build_path_data, to_diagnostic_svg, to_segment_diagnostic_svg, to_svg};
+pub use thr::{ThrMetadata, to_thr};