dev: LLVM source-based code coverage support

[cdecker]

Summary

Add support for LLVM source-based code coverage instrumentation, enabling developers and CI to generate coverage reports for the Core Lightning codebase.

Changes

Build System

• Makefile: Add coverage-clang-collect and coverage-clang-report targets for collecting and generating coverage reports
• Makefile: Fix missing endif for PYTEST_TESTS conditional
• Makefile: Respect CARGO_TARGET_DIR environment variable for Rust builds

Coverage Scripts (contrib/coverage/)

• collect-coverage.sh: Merges .profraw files into a single .profdata file with validation, parallel processing, and batch handling for large file counts
• generate-coverage-report.sh: Generates HTML reports using llvm-cov show for all instrumented binaries

CI Workflow

• coverage.yaml: Daily workflow that builds with coverage instrumentation, runs tests, and publishes HTML reports to GitHub Pages

Usage

# Configure with coverage enabled
./configure --enable-coverage CC=clang

# Build
make -j$(nproc)

# Run tests with coverage collection
CLN_COVERAGE_DIR=/tmp/cln-coverage make pytest

# Generate coverage report
make coverage-clang
# Report available at coverage/html/index.html

Notes

• Uses LLVM's source-based coverage instrumentation (-fprofile-instr-generate -fcoverage-mapping)
• Handles CLN's multi-process architecture by collecting per-test .profraw files
• Coverage scripts validate and filter corrupt/incomplete profile files before merging

[cdecker]

@rustyrussell sorry for forgetting the script files in the coverage PR. It was being covered by .gitignore so I didn't see them when staging the content.