[M5] add cargoxx add/remove (linkdb-validated)

CHANGELOG.md

@@ -78,6 +78,17 @@ All notable changes to cargoxx will be documented in this file.
   and `[build]` honoring `warnings_as_errors` and `sanitizers`. Source
   paths emitted relative to `build/` (i.e. prefixed with `../`).
   Output is deterministic. `tests/codegen_cmake.cpp` covers 11 cases.
+- `cargoxx add <pkg>@<version> [--components a,b]` edits `Cargoxx.toml`,
+  validates the new dep against the curated linkdb (so unknown packages
+  and missing components fail before any disk write), and rejects
+  already-declared deps. Auto-resolution against nixhub.io/lazamar is
+  deferred — for v0.1 the version is required.
+- `cargoxx remove <pkg>` drops a declared dep from `Cargoxx.toml`,
+  errors when the dep is not declared. Other deps are preserved.
+- End-to-end verified on a freshly-scaffolded project. `tests/cmd_add.cpp`
+  and `tests/cmd_remove.cpp` cover 9 cases. Known cosmetic issue:
+  toml++ writes top-level sections alphabetically, so `[package]` may
+  end up after `[dependencies]` post-mutation; logged for M6 polish.
 - `cargoxx run [--release] [--bin <name>] [-- <args>...]`,
   `cargoxx test [--release]`, and `cargoxx clean`. `run` builds first,
   picks the binary (errors with the available list when the project