[M6] README: reflect M6 state (self-hosting + 6-stage resolver chain)

README.md

@@ -1,23 +1,104 @@
 # cargoxx
 
-A Cargo-style frontend for modern C++ that uses Nix as the source of truth for
-dependencies and generates CMake for the build. Users author projects with C++23
-modules and never touch CMake or `flake.nix` by hand.
+A Cargo-style frontend for modern C++ that uses Nix as the dependency
+source of truth and generates CMake for the build. Users author projects
+with C++23 modules and never touch CMake or `flake.nix` by hand.
 
-> Status: pre-v0.1, milestone M0 (repo skeleton). Not yet usable.
+> Status: M6. Self-hosting (cargoxx builds itself from `Cargoxx.toml`).
+> Full auto-resolver chain (Conan / vcpkg / nix-config / CMake FindModule /
+> pkg-config / brute-force fallback). Pre-v0.1; CLI is stable but the
+> on-disk linkdb schema is not.
 
 See [`SPEC.md`](./SPEC.md) for the user-facing contract and
-[`TECH_SPEC.md`](./TECH_SPEC.md) for the implementation plan.
+[`TECH_SPEC.md`](./TECH_SPEC.md) for the implementation.
+
+## What it does
+
+```sh
+cargoxx new myapp        # scaffold Cargoxx.toml + src/main.cpp
+cd myapp
+cargoxx add fmt          # resolve fmt → write recipe to ~/.cache/cargoxx/linkdb.sqlite,
+                         # pin nixpkgs rev into Cargoxx.lock
+cargoxx build            # generate build/CMakeLists.txt + flake.nix,
+                         # then `nix develop -c cmake --build`
+cargoxx run              # build + exec
+cargoxx test             # build + ctest
+```
+
+The user never touches `build/CMakeLists.txt` or `flake.nix`. Both are
+regenerated from `Cargoxx.toml` on every build.
+
+## How dependency resolution works
+
+`cargoxx add <pkg>` (and `cargoxx build` on a missing dep) walks a probe
+chain. The first probe whose verify-link build succeeds wins; its recipe
+is confirmed in the SQLite overlay.
+
+| # | Probe | Source |
+|---|---|---|
+| 1 | conan-center-index | exact + Levenshtein fuzzy ≤ ⌈len/4⌉ |
+| 2 | microsoft/vcpkg ports | exact + Levenshtein fuzzy |
+| 3 | nix-probe | `<X>Config.cmake` under `pkgs.<pkg>.dev` |
+| 4 | cmake-findmodule | CMake's bundled `Find*.cmake` |
+| 5 | pkg-config | `<X>.pc` under `pkgs.<pkg>.dev/lib/pkgconfig` |
+| 6 | brute-force | every `lib/lib*.{a,so,dylib}` + `include/` |
+
+Package names match nixpkgs/devbox. Conan/vcpkg fuzzy match is internal
+only — the user-typed name is preserved end-to-end in the manifest,
+lockfile, and linkdb key.
+
+### Debugging a failed `cargoxx add`
+
+Every probe's verify-link scratch project is preserved under
+`~/.cache/cargoxx/last-failure/<pkg>/<NN>-<probe>/`. Re-run cmake inside
+any subdir to reproduce, or just inspect the generated `flake.nix` and
+`build/CMakeLists.txt`.
+
+```
+~/.cache/cargoxx/last-failure/sqlite/
+  01-vcpkg/              # vcpkg fuzzy candidate, failed
+  02-cmake-findmodule/   # FindSQLite3 candidate, won
+```
 
 ## Building from source
 
-cargoxx builds inside a Nix-managed dev shell that pins the toolchain
-(Clang with C++23 modules + `import std;`, CMake ≥ 3.30, Ninja).
+cargoxx is self-hosted, so the bootstrap path requires an existing
+cargoxx binary (release artifact or one built from a prior commit) plus
+a Nix toolchain.
 
 ```sh
 nix develop
-cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Debug
-cmake --build build
+cargoxx build             # regenerate build/CMakeLists.txt + root flake.nix
+cmake -S build -B build/debug -G Ninja
+cmake --build build/debug
 ```
 
-The resulting binary is at `build/cargoxx`.
+The dev shell ships `pkgs.gcc15Stdenv` (g++ 15, libstdc++, with
+`import std;` via `bits/std.cc`). cmake/ninja are also injected.
+
+On a clean clone with an empty `~/.cache/cargoxx/linkdb.sqlite`,
+`cargoxx build` auto-resolves all three of cargoxx's own deps
+(`sqlite`, `reproc`, `catch2_3`) on first invocation.
+
+## Manifest
+
+```toml
+[package]
+name = "myapp"
+version = "0.1.0"
+edition = "cpp23"
+
+[dependencies]
+fmt = "10.2"
+sqlite = "*"
+
+[dev-dependencies]
+catch2_3 = "*"
+
+[build]
+warnings_as_errors = true
+include_dirs = ["third_party"]
+sanitizers = ["address"]
+```
+
+See [`SPEC.md`](./SPEC.md) §4 for the full schema.