#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

crates/format/src/lib.rs

@@ -145,6 +145,19 @@ fn preview_mime(bytes: &[u8]) -> &'static str {
 /// The JSON of a file and the data after it (v4), split at [`BLOB_MAGIC`] after the first JSON
 /// value.
 fn split_tail(text: &[u8]) -> (&[u8], &[u8]) {
+    // JSON holds no raw NUL, so the marker (which has one) starts at the first NUL, if the file
+    // has one. The writer puts it right after the JSON's last byte; found there, it is where the
+    // first JSON value ends, without parsing the JSON twice. Anything else takes the parse.
+    let Some(nul) = text.iter().position(|&b| b == 0) else {
+        // No NUL, so no marker: the parse below would find none either.
+        return (text, &[]);
+    };
+    if let Some(at) = nul.checked_sub(1)
+        && text.get(at..).is_some_and(|r| r.starts_with(BLOB_MAGIC))
+        && at.checked_sub(1).and_then(|k| text.get(k)).is_some_and(|b| matches!(b, b'}' | b']' | b'"'))
+    {
+        return (text.get(..at).unwrap_or(text), text.get(at + BLOB_MAGIC.len()..).unwrap_or_default());
+    }
     let mut values = serde_json::Deserializer::from_slice(text).into_iter::<serde::de::IgnoredAny>();
     if values.next().is_some_and(|v| v.is_ok()) {
         let end = values.byte_offset();