#1172: Pathfinder Outline 19x, SVG export 1.5x and open 1.2x faster, same output

At 50,000 paths unless noted, against main:

- Pathfinder Outline (300 paths): 145 → 7.5 ms. Each segment was tested against every junction
  of the arrangement, and each chain against every chain kept so far. Now a segment tests only
  the junctions inside its control box (junctions are sorted by x; a point within reach of a
  curve is within reach of that box), with the same nearest-point test. Coincident chains are
  found through a grid on their midpoints, taking the first match in the same order a scan does.
- export SVG: 267 → 176 ms. Path data is written straight into one string (`push_num`, the
  same digits and trimming as `fmt_num`) instead of a string per number, point and segment.
- open .vectorcraft: 239 → 195 ms. The JSON was parsed once only to find where it ends:
  - A file with no NUL has no data after it.
  - Otherwise the marker (which holds the first NUL, as JSON can't) is taken when it directly
    follows the JSON's last byte, as the writer puts it.
  - Anything else takes the old parse.

`perf --deep` prints output hashes of an SVG export and a save → open → save round trip; main
and this branch print the same ones. pathfinder_pinned (Outline included) passes, and so do the
pathops, format and svg suites (345 tests).

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01VmMmc1J8XeNLxhRihWv9cc

apps/vectorcraft-cli/src/perf.rs

@@ -207,6 +207,13 @@ fn deep(n: usize) -> Result<(), String> {
         }),
     ));
     note("after typing");
+    // Output hashes, to compare builds: the SVG export and a save → open → save round trip.
+    let fnv = |b: &[u8]| b.iter().fold(0xcbf2_9ce4_8422_2325u64, |h, &x| (h ^ u64::from(x)).wrapping_mul(0x100_0000_01b3));
+    let doc = synthetic(n)?;
+    let svg = vectorcraft_svg::export(&doc, &vectorcraft_svg::ExportOptions::default());
+    let saved = vectorcraft_format::save_file(&doc);
+    let reopened = vectorcraft_format::load(&saved).map_err(|e| e.to_string())?;
+    outln!("  output hash: SVG export {:016x}, save→open→save {:016x}", fnv(svg.as_bytes()), fnv(&vectorcraft_format::save_file(&reopened)));
     outln!("  tracked (no budget yet)");
     for (name, ms) in &rows {
         outln!("  {name:<52} {ms:>10.2} ms");