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/lib.rs

@@ -10,5 +10,7 @@
 pub mod args;
 pub mod benches;
 pub mod benchmark;
+#[cfg(target_vendor = "apple")]
+pub mod signpost;
 mod simd_width;
 pub mod synthetic;