Add profiling build target + native profiler workflow; closes #345

The default Release build (-O3 -DNDEBUG) omits frame pointers and ships no
DWARF, so it can't be line-annotated or unwound; build-debug (-O0) profiles
unoptimized code. Add a dedicated profiling variant for C++ hot-path work
(validating #331/#342/#343/#344).

- CMakeLists.txt: MCRF_PROFILE option adds -fno-omit-frame-pointer on top of
  RelWithDebInfo (-O2 -g). Binary not stripped. Post-build lib/assets/scripts
  copy already keys on the mcrogueface target, so build-profile/ self-populates.
- Makefile: `make profile` (-> build-profile/), `make callgrind SCRIPT=...`
  (one-shot Callgrind on a headless benchmark), `make clean-profile` (wired
  into clean-all).
- docs/profiling.md: full Callgrind + perf workflow, incl. the one-time
  `sudo sysctl kernel.perf_event_paranoid=1` perf needs on this box.
- Pointers from the Makefile header and CLAUDE.md build section.

Verified: DWARF sections present, frame-pointer prologues emitted, headless
smoke test passes, Callgrind yields source line + call-count attribution, and
perf --call-graph fp resolves engine symbols.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/profiling.md

@@ -0,0 +1,82 @@
+# Native Profiling Workflow (Callgrind + perf)
+
+External C++ profiling for engine hot-path work (issue #345). This is separate from
+the in-engine `ProfilerOverlay` / `ProfilingMetrics` live HUD (`src/Profiler.*`,
+`GameEngine.h`) — those show frame time / draw calls at runtime; this is for
+diagnosing and A/B-validating C++ optimizations (e.g. #331, #342, #343, #344).
+
+## The profiling build
+
+The default `make` (Release, `-O3 -DNDEBUG`) omits frame pointers and ships no DWARF,
+so it can't be line-annotated or unwound. `make build-debug` (`-O0`) profiles the wrong
+(unoptimized) code. Use the dedicated profiling build instead:
+
+```bash
+make profile
+```
+
+Produces `build-profile/mcrogueface`, configured `RelWithDebInfo` (`-O2 -g`) plus
+`-fno-omit-frame-pointer` (CMake `-DMCRF_PROFILE=ON`). Not stripped. The build is
+self-contained — `lib/`, `assets/`, and `scripts/` are copied next to the binary just
+like the normal build, so `--headless --exec` works out of the box.
+
+## Callgrind — primary, deterministic, no special permissions
+
+Best fit for validating optimizations. `--headless --exec <script>` is deterministic,
+so Callgrind yields exact, reproducible instruction counts (`Ir`) with source file:line
+attribution and call counts. ~30-50x slowdown, so point it at a bounded benchmark.
+
+One-shot via the Makefile (defaults to the #331 benchmark; override with `SCRIPT=`):
+
+```bash
+make callgrind SCRIPT=tests/benchmarks/issue_331_property_read_bench.py
+callgrind_annotate build-profile/callgrind.out | head -60
+```
+
+Or manually:
+
+```bash
+valgrind --tool=callgrind --callgrind-out-file=build-profile/callgrind.out \
+  build-profile/mcrogueface --headless --exec tests/benchmarks/<bench>.py
+
+# Function-level, engine code only:
+callgrind_annotate --threshold=95 build-profile/callgrind.out \
+  | grep -E "src/" | grep -Ev "python3\.14|/usr/|libpython"
+```
+
+**A/B a change:** run Callgrind before and after, compare the total `Ir` for the target
+function (Callgrind reports per-function self + inclusive counts and per-call-site
+counts). Because it's deterministic, any delta is the change, not noise. `kcachegrind`
+(GUI) isn't installed on the dev box; `callgrind_annotate` (CLI) is sufficient.
+
+Caveat: Callgrind models an idealized cache and counts instructions, not wall-clock —
+great for "did this do less work," not for real-time behavior.
+
+## perf — real wall-clock sampling
+
+For the interactive game where you need actual time spent (draw calls, SFML/Python cost
+Callgrind's model hides):
+
+```bash
+perf record --call-graph fp -- build-profile/mcrogueface [args]
+perf report
+```
+
+`--call-graph fp` relies on the frame pointers the profiling build retains.
+
+**Prerequisite (one-time, needs root):** the dev box ships
+`kernel.perf_event_paranoid = 3`, which blocks `perf_event_open` for non-root entirely.
+Lower it for the session:
+
+```bash
+sudo sysctl kernel.perf_event_paranoid=1
+```
+
+(Or run perf under `sudo`. To persist, add `kernel.perf_event_paranoid = 1` to
+`/etc/sysctl.conf`.)
+
+## Notes
+
+- gprof is intentionally not used: it requires a `-pg` instrumentation recompile and
+  handles the Python/SFML shared libraries poorly.
+- Clean up with `make clean-profile` (also covered by `make clean-all`).