ci: migrate coverage from tarpaulin to cargo-llvm-cov

[acgetchell]

Summary

Migrate local and CI coverage generation from cargo-tarpaulin to cargo-llvm-cov.

This is the first pass before doing the same migration in acgetchell/delaunay, so this issue should capture the concrete command, workflow, and reporting decisions that transfer cleanly to downstream crates.

Motivation

cargo-llvm-cov should give us coverage based on Rust's LLVM instrumentation, with fewer tarpaulin-specific discrepancies between local and CI runs. Doing this in la-stack first keeps the experiment smaller before applying the same pattern to the larger delaunay coverage workflow.

Proposed Change

• Add cargo-llvm-cov installation/caching to the coverage workflow.
• Replace tarpaulin invocations in local coverage recipes and CI with equivalent cargo llvm-cov commands.
• Preserve generated report formats needed by coverage consumers:
  • local HTML report for developers
  • Cobertura XML or compatible output for Codecov/Codacy, if applicable
• Preserve relevant exclusions or document intentional behavior changes.
• Update docs that mention tarpaulin commands or report paths.
• Remove stale tarpaulin install/check logic once the replacement is validated.

Acceptance Criteria

• Local coverage generation works with cargo-llvm-cov.
• CI coverage generation works with cargo-llvm-cov.
• Coverage upload artifacts remain compatible with configured reporting services.
• Documentation no longer points developers at tarpaulin-specific commands or paths except in archive/history docs.
• Lessons learned are captured so acgetchell/delaunay#343 can reuse the migration pattern.

Related

Blocks acgetchell/delaunay#343.