feat: add coverage and command doc rule

Add Coverage.cmake with gcovr integration, detecting Clang vs
GCC and wrapping llvm-cov gcov in a build-dir script to handle
paths with spaces on Windows. Update README and AGENTS.md with
the new one-command-per-block documentation rule.

.gitignore

@@ -2,6 +2,7 @@
 build
 build-*
 cmake-build-*
+coverage
 
 # Editors
 .vscode

AGENTS.md

@@ -12,13 +12,44 @@
 
 ### Commands
 
+Configure (default):
 ```sh
 cmake -S . -B build -G Ninja
-ninja -C build          # build
+```
-ninja -C build check    # run tests (full Unity output, colored)
+
-ninja -C build test     # run tests (CTest summary only)
+Configure with coverage:
-./build/main            # run (Linux/macOS)
+```sh
-./build/main.exe        # run (Windows)
+cmake -S . -B build-cov -G Ninja -DENABLE_COVERAGE=ON
+```
+
+Build:
+```sh
+ninja -C build
+```
+
+Run tests — full Unity output, colored:
+```sh
+ninja -C build check
+```
+
+Run tests — CTest summary only:
+```sh
+ninja -C build test
+```
+
+Run tests and generate coverage HTML:
+```sh
+ninja -C build-cov coverage
+```
+
+Run (Linux/macOS):
+```sh
+./build/main
+```
+
+Run (Windows):
+```sh
+./build/main.exe
 ```
 
 ### Dependencies
@@ -114,6 +145,15 @@ in HH:MM:SS.mmm format, updating every 10ms with color output.
   `-` or numbered lists, fenced code blocks with language hints
   (```` ```cpp ````, ```` ```sh ````).
 - Keep examples concise, up-to-date, and self-documenting.
+- Each shell command gets its own fenced code block — do **not** combine
+  multiple commands into one block. Precede each block with a short
+  plain-text label describing what the command does:
+
+  Setup build:
+  ```sh
+  cmake -S . -B build -G Ninja
+  ```
+
 - This file (`AGENTS.md`) follows its own rules.
 
 ## Source Layout

CMakeLists.txt

@@ -9,6 +9,8 @@ list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/deps")
 include(Platform)
 include(Flags)
 
+include(Coverage)
+
 # Example Library
 add_subdirectory(ctdd)
 

README.md

@@ -10,37 +10,74 @@ manual installation required beyond the tools listed below.
 ## Requirements
 
 | Tool         | Purpose                                               |
-| ------------------- | --------------------- |
+| ------------ | ----------------------------------------------------- |
 | CMake ≥ 3.21 | Build system                                          |
 | Ninja        | Build backend                                         |
 | C23 compiler | GCC 14+, Clang 18+                                    |
 | Ruby ≥ 3.0   | CMock mock generation                                 |
+| gcovr ≥ 6.0  | Coverage reports — optional (`uv tool install gcovr`) |
 
 ## Build
 
+Configure:
 ```sh
 cmake -S . -B build -G Ninja
+```
+
+Build:
+```sh
 ninja -C build
 ```
 
 ## Test
 
+Full Unity output with colors:
 ```sh
-ninja -C build check         # full Unity output, colored
+ninja -C build check
-ninja -C build test          # CTest summary only
+```
+
+CTest summary only:
+```sh
+ninja -C build test
 ```
 
 `check` builds all suites and runs CTest with `--output-on-failure`,
 so assertion-level detail appears on any failure without running
 binaries by hand.
 
-## Run
+## Coverage
 
+Configure with coverage instrumentation:
 ```sh
-./build/main.exe             # Windows
+cmake -S . -B build-cov -G Ninja -DENABLE_COVERAGE=ON
-./build/main                 # Linux / macOS
 ```
 
+Generate the HTML report:
+```sh
+ninja -C build-cov coverage
+```
+
+Open `build-cov/coverage/index.html` in a browser to view results.
+
+Only `ctdd/` source files are measured — Unity, CMock, and generated
+mock files are excluded. Requires GCC or Clang with gcov support, and
+`gcovr` on `PATH`.
+
+> **Windows note:** requires GCC (e.g. `scoop install gcc`) or a Clang
+> build that includes compiler-rt. A custom Clang without compiler-rt
+> will fail at link time.
+
+## Run
+
+Windows:
+```sh
+./build/main.exe
+```
+
+Linux / macOS:
+```sh
+./build/main
+```
 
 ## Adding a new module (TDD workflow)
 

deps/Coverage.cmake

@@ -0,0 +1,46 @@
+# ==============================================================================
+# Coverage
+# ==============================================================================
+# gcov-based coverage via --coverage, reported by gcovr.
+# Works with GCC and Clang on Linux and macOS out of the box.
+# Windows requires GCC (e.g. MinGW via scoop install gcc) or a Clang build
+# that includes compiler-rt (clang_rt.profile).
+#
+# Requires: gcovr on PATH (install via: uv tool install gcovr)
+# Usage:    cmake -DENABLE_COVERAGE=ON ...  then  ninja coverage
+# Report:   build/coverage/index.html
+# ==============================================================================
+
+option(ENABLE_COVERAGE "Build with coverage instrumentation" OFF)
+
+if (ENABLE_COVERAGE)
+    if (NOT CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
+        message(FATAL_ERROR "ENABLE_COVERAGE requires GCC or Clang")
+    endif()
+
+    find_program(GCOVR_EXE gcovr REQUIRED)
+
+    if (CMAKE_C_COMPILER_ID MATCHES "Clang")
+        find_program(LLVM_COV_EXE llvm-cov REQUIRED)
+        # gcovr needs a single-token gcov executable. Wrap llvm-cov gcov in a
+        # script placed in the build dir (guaranteed no spaces in path).
+        if (WIN32)
+            set(GCOV_EXECUTABLE "${CMAKE_BINARY_DIR}/llvm-gcov.bat")
+            file(WRITE "${GCOV_EXECUTABLE}" "@echo off\r\n\"${LLVM_COV_EXE}\" gcov %*\r\n")
+        else()
+            set(GCOV_EXECUTABLE "${CMAKE_BINARY_DIR}/llvm-gcov.sh")
+            file(WRITE "${GCOV_EXECUTABLE}" "#!/bin/sh\nexec \"${LLVM_COV_EXE}\" gcov \"$@\"\n")
+            file(CHMOD "${GCOV_EXECUTABLE}"
+                PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE
+                            GROUP_READ GROUP_EXECUTE
+                            WORLD_READ WORLD_EXECUTE)
+        endif()
+        message(STATUS "Coverage: enabled (gcovr: ${GCOVR_EXE}, gcov: ${LLVM_COV_EXE} gcov)")
+    else()
+        set(GCOV_EXECUTABLE "gcov")
+        message(STATUS "Coverage: enabled (gcovr: ${GCOVR_EXE})")
+    endif()
+
+    add_compile_options(--coverage -O0 -g)
+    add_link_options(--coverage)
+endif()

tests/CMakeLists.txt

@@ -67,3 +67,27 @@ add_custom_target(check
     USES_TERMINAL
     DEPENDS ${TEST_TARGETS}
 )
+
+if (ENABLE_COVERAGE)
+    set(COVERAGE_DIR "${CMAKE_BINARY_DIR}/coverage")
+    add_custom_target(coverage
+        COMMAND ${CMAKE_CTEST_COMMAND} --output-on-failure
+        COMMAND ${CMAKE_COMMAND} -E make_directory "${COVERAGE_DIR}"
+        COMMAND ${GCOVR_EXE}
+                --gcov-executable "${GCOV_EXECUTABLE}"
+                --root "${CMAKE_SOURCE_DIR}"
+                --filter "${CMAKE_SOURCE_DIR}/ctdd/"
+                --exclude "${CMAKE_SOURCE_DIR}/tests/"
+                --exclude ".*Mock.*"
+                --exclude ".*unity.*"
+                --exclude ".*cmock.*"
+                --html-details "${COVERAGE_DIR}/index.html"
+                --txt
+                --print-summary
+                "${CMAKE_BINARY_DIR}"
+        WORKING_DIRECTORY "${CMAKE_BINARY_DIR}"
+        USES_TERMINAL
+        DEPENDS ${TEST_TARGETS}
+        COMMENT "Coverage report: ${COVERAGE_DIR}/index.html"
+    )
+endif()