📝 rewrite README and fix setup script tool installs 📝

- README rewritten from 137 → 439 lines: full tool tables, env var reference, volume/cache guide, cross-compile target matrix, sccache remote backend docs, BuildKit cache mount info
- Update image name from `casjaysdevdocker/rust` to `casjaysdev/rust` throughout
- Add `clang-dev`, `llvm-dev`, `openssl-libs-static` to build deps; add `perf` (non-fatal) for flamegraph/samply support
- Fix rustup nightly component flag syntax (split into two `--component` flags)
- Move `cargo-nextest` to fallback block with `--locked`; rename `typos` → `typos-cli` and `cargo-flamegraph` → `flamegraph` to match correct crate names

README.md
rootfs/root/docker/setup/05-custom.sh

README.md

@@ -188,7 +188,12 @@ Run miri with: `cargo +nightly miri test`
 | `cargo-criterion` | Criterion-based benchmark runner with statistical analysis |
 | `hyperfine` | Command-line benchmarking tool — wall-clock timing with statistics |
 | `samply` | Sampling profiler — records perf profiles, opens in Firefox Profiler |
-| `cargo-flamegraph` | Generate flame graphs from `cargo bench` or any cargo command |
+| `flamegraph` | Generate flame graphs from `cargo bench` or any cargo command (`cargo flamegraph`) |
+
+> **Perf note:** `perf` is installed from the Alpine package repo. Profiling with
+> `samply` or `flamegraph` inside Docker requires elevated capabilities at runtime:
+> `--privileged` or `--cap-add SYS_ADMIN --cap-add PERFMON`. This is a host grant
+> — it cannot be baked into the image.
 
 ### Fuzzing