Speed up cold document geometry rebuilds

docs/perf.md

@@ -1,5 +1,43 @@
 # Design loading
 
+## Cold geometry rebuilds after 0.3.1
+
+Three alternating cold-process comparisons on the same Windows machine and optimized
+`ci` profile, with no concurrent build, compare the unchanged 0.3.1 source with the
+straight-line projection and exact surface-refinement reuse changes:
+
+| Saved design | Before, median evaluation | After | Reduction |
+|---|---:|---:|---:|
+| Public sensor enclosure, 21 features, 52 KB | 11.96 s | 8.85 s | 26.0% |
+| Local Pi case assembly, 25 features, 2,990 bodies | 14.75 s | 11.57 s | 21.6% |
+
+These are geometry rebuild times, not complete desktop opening times. The public fixture
+can be generated with `solvecraft-cli run examples/acceptance/sensor_enclosure.json
+--save sensor.solvecraft --quiet`, then measured with `solvecraft-cli bench-open
+sensor.solvecraft`. Each benchmark starts a new process; this does not measure a warm
+timeline-cache hit. The local assembly is private and is not part of the repository.
+
+Straight segments now use guarded algebraic closest-point projection instead of sampling
+65 points and running Newton iterations. Other curve types retain their existing behavior.
+Curved-face refinement reuses derivative and midpoint evaluations with exact finite input
+keys and bounded, local caches. It keeps the original tolerance, seam and singularity
+checks, refinement passes, and triangle limits.
+
+Identity assembly placements retain the definition body's shared display-mesh cache.
+Changed placements continue to build separate meshes in their own coordinates. Face-name
+preparation remains eager during placement: deferring it merely shifted expensive work to
+thumbnails and the first face query, so that approach was excluded.
+
+Both saved fixtures retain exactly the same body names, face/edge/vertex/shell counts,
+bounds, volumes, and surface areas. Focused tests compare cached and uncached refinement
+coordinates and normals bit for bit on periodic, singular and transformed surfaces.
+
+For desktop phase profiling, set `SOLVECRAFT_PROFILE_OPEN=1` before launching the app.
+Interactive Open reports geometry, placement, tessellation, thumbnail and total worker
+times to stderr without logging the source path. Thumbnails were about 0.1 seconds of
+the slow assembly's open and were not the main bottleneck. Further cold Boolean and
+import costs remain; these changes do not make all documents open instantly.
+
 `solvecraft-cli bench-open design.solvecraft` reports JSON lines for file reading,
 JSON decoding and migrations (including input-name repair), timeline evaluation, individual
 features, and world placement/display meshing. `--parse-only` stops before geometry evaluation.