fix(ci): force tarpaulin LLVM engine to end the recurring coverage segfault (#163)

## Summary

Fixes the **recurring `rust-ci / Coverage (tarpaulin + codecov)` red**
(failed on #161,
#162, …) with a **locally-verified** repo-side change.

**Root cause:** the standards rust-ci reusable runs `cargo tarpaulin
--out Xml` with the
default **ptrace** engine, which **segfaults** on the CI runners while
instrumenting the
property-test binary (`"Failed to run tests: A segfault occurred while
executing tests"`).
It's a tarpaulin instrumentation flake — the suite passes under `cargo
test`, and the
ptrace engine doesn't even reproduce the segfault locally.

**Fix:** `tarpaulin.toml` selecting the **LLVM (source-based) engine**,
which avoids
ptrace entirely. The reusable's `cargo tarpaulin --out Xml` reads this
`[default]`
profile, so **no standards-reusable change is needed**.

**Verified locally** (I installed tarpaulin and reproduced the setup):
- default engine locally → passes (segfault is CI-runner-specific);
- `[default] engine = "Llvm"` + `cargo tarpaulin --out Xml` → activates
`LLVM_PROFILE_FILE` coverage across 26 test binaries, exits 0, writes a
valid
  `cobertura.xml`.
- Note: the value must be capitalized `"Llvm"` — serde is
case-sensitive; lowercase
  silently falls back to ptrace.

This clears the last **repo-side-fixable** CI red. Remaining reds are
genuinely external:
`governance / Validate Hypatia Baseline` is pre-existing **#41 debt**
(needs the hypatia
CLI, out of session scope, or a baseline-regen decision), which the
owner has merged past.

## FLAGS (unchanged)
- `gossamer` + `conative-gating` still 403-blocked → **#83 / #103
staged** until added to
  the environment's repo scope.
- OPEN proof obligations 1.1 / 1.2 (formal) / 3.2 remain honestly
staged.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_0172RBMz3qYjb1ttzD2i7RNh

---
_Generated by [Claude
Code](https://claude.ai/code/session_0172RBMz3qYjb1ttzD2i7RNh)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

tarpaulin.toml

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: MPL-2.0
+# cargo-tarpaulin configuration.
+#
+# Force the LLVM (source-based) coverage engine. The default ptrace engine
+# segfaults on the CI runners while instrumenting the property-test binary
+# ("Failed to run tests: A segfault occurred while executing tests") — a known
+# cargo-tarpaulin ptrace flake, not a real test failure (the suite passes under
+# `cargo test`, and the ptrace engine does not even reproduce the segfault
+# locally). The LLVM engine avoids ptrace entirely.
+#
+# The standards rust-ci reusable runs `cargo tarpaulin --out Xml`, which reads
+# this [default] profile, so the engine switch needs no reusable change. Verified
+# locally: `cargo tarpaulin --out Xml` then activates LLVM_PROFILE_FILE coverage
+# and produces cobertura.xml cleanly (exit 0).
+[default]
+engine = "Llvm"