Add Clang coverage pass integrated with test build flow

[ggeier]

Motivation

• Provide an opt-in way to generate Clang/LLVM coverage from the project's test suite and export LCOV output for CI and local analysis.
• Keep the instrumentation opt-in and guarded to avoid affecting regular builds and to require a Clang toolchain when enabled.

Description

• Add a new CMake option COLDTRACE_ENABLE_COVERAGE to enable Clang coverage instrumentation and enforce Clang when enabled, and disable IPO while coverage is active.
• When enabled, add compiler/link flags -fprofile-instr-generate and -fcoverage-mapping, and wire llvm-profdata/llvm-cov usage to generate reports.
• Collect test executable targets into a global property COLDTRACE_TEST_TARGETS in test/CMakeLists.txt and use that list to build coverage object lists for reporting.
• Add a coverage custom target that runs the test suite via ctest with LLVM_PROFILE_FILE, merges profiles via llvm-profdata, emits a summary via llvm-cov report, and exports LCOV to coverage/coverage.lcov, and document usage in README.md.

Testing

• Ran configuration with cmake -S . -B build-coverage -DCMAKE_BUILD_TYPE=Debug -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCOLDTRACE_ENABLE_COVERAGE=ON, which configured successfully.
• Attempted to build the coverage target with cmake --build build-coverage --target coverage -j4, which failed during linking in this environment due to Swift/Clang TSAN runtime / libdispatch linker incompatibilities unrelated to the coverage wiring; the failure prevented completing runtime test execution and profile merging.

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Codex Task