fix: C allocator contract, shadertoy overloads/MSL stride, minidumps

Closes the remaining library rows of the final completeness audit
(docs/rewrite/notes/final-Z3.md):
- libghostty-vt: OSC parser storage goes through the caller-supplied C
  allocator via a ByteStorage abstraction; ghostty_osc_new reports
  GHOSTTY_OUT_OF_MEMORY and failed grows invalidate the OSC like Zig.
  Tested with a counting/failing/limiting C allocator.
- shadertoy: sampler parameter rewriting is keyed by full signature so
  overloads work; the Metal uniform block keeps std140 strides for
  iChannelTime[4] (MSL offsets checked against the upload layout).
- tripwire.zig ported (named trip points, hit counts, verification) and
  used to port the previously skipped allocation-failure tests that have
  a fallible Rust path; the rest are documented.
- crash: fatal signals capture siginfo + registers and a forked helper
  writes a minidump (minidump-writer) as an envelope attachment, like
  sentry-native/Breakpad; tests parse it with the minidump crate.
- kitty LoadingImage::debug_dump (debug builds) and the Darwin
  Instruments signpost around benchmark runs.

Test Plan:
- cargo test -p ghostty-vt-capi --test c_allocator
- cargo test -p ghostty-crash --test minidump

crates/ghostty-bench/src/benchmark.rs

@@ -49,6 +49,16 @@ pub fn run(b: &mut dyn Benchmark, mode: RunMode) -> Result<RunResult> {
 
 fn run_steps(b: &mut dyn Benchmark, mode: RunMode) -> Result<RunResult> {
     let mut result = RunResult::default();
+
+    // On Darwin, emit an Instruments signpost interval around the run so the
+    // results are easy to find in Instruments (ends when dropped).
+    #[cfg(target_vendor = "apple")]
+    let _signpost = crate::signpost::Interval::begin(
+        c"com.mitchellh.ghostty",
+        c"ghostty",
+        (&raw const *b).cast::<()>() as usize,
+    );
+
     let start = Instant::now();
     loop {
         b.step()?;