phase3: coz integration status; cjson C pipeline proven end-to-end

- coz wired (harness -DHOTPATH_COZ hook, scripts, just coz) + installed,
  but the apt coz runtime aborts in its own init_coz/pthread_create
  interposition on this glibc (known coz bug, reproduces on the pristine
  binary). Documented in results/cjson/coz/README.md; callgrind cache-sim
  is the working profiler.
- cjson no-op verdict smoke: full C pipeline flows (clang {out} baseline
  rebuild, Unity/CTest + golden gates, interleaved bench) to a correct
  null result (median 0.9995).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2

results/cjson/coz/README.md

@@ -0,0 +1,23 @@
+# coz on cjson — integration status
+
+coz is **wired** into the harness (SPEC §12): `targets/cjson/harness.c`
+carries a `-DHOTPATH_COZ` throughput progress point, and
+`scripts/coz-profile.sh` + `just coz cjson` build the instrumented binary
+and run `coz run`. `scripts/coz-summary.py` ranks lines by causal
+throughput effect.
+
+**Runtime limitation in this container:** the distro `coz-profiler`
+(apt) aborts inside its own `init_coz` during startup —
+`pthread_create` interposition recurses ("init_coz in progress, do not
+recurse" → SIGABRT) before `main` runs. This is a known coz/glibc
+incompatibility on newer glibc, independent of the harness. It reproduces
+on the pristine binary with a bare `coz run`, so it is not our
+instrumentation.
+
+**What we use instead:** callgrind `--cache-sim --branch-sim`
+(`results/cjson/hotspots.txt`) is the working profiler and gives an
+unambiguous class-selection signal for cjson — glibc float
+serialization/parsing (`__printf_fp`, `strtod`) plus cJSON
+`parse_value`/`print_value` dominate. The coz path is ready for an
+environment with a compatible coz runtime (bench-metal / a pinned coz
+build); no code change is needed there.