refactor: slim flake.nix into nix/ entry files

nix/rust.nix

@@ -0,0 +1,56 @@
+# Shared crane wiring for the rust workspace: the cargo source
+# filter, the once-built dependency artifacts, and the native build
+# inputs every rust derivation (packages + checks) consumes.
+# Imported per system from flake.nix.
+{ pkgs, craneLib }:
+rec {
+  # Shared between buildDepsOnly + buildPackage + cargoClippy/cargoTest
+  # so every derivation in the chain sees the same toolchain shape.
+  # git: hive-c0re's `lifecycle::tests::setup_proposed_*` shell out to
+  # `git init` + commit under `cargo test` in the sandbox.
+  # sqlite: matrix-sdk's `sqlite` feature (`hive-matrix-mcp` workspace
+  # member) — the matrix-sdk-sqlite + rusqlite stack links against
+  # system libsqlite3 by default.
+  # cmake: builds `aws-lc-sys` (BoringSSL) from source — pulled in by
+  # the `rustls` (aws-lc-rs) crypto provider under the OTLP/reqwest
+  # stack in `hive-metric`.
+  nativeBuildInputs = [
+    pkgs.git
+    pkgs.sqlite
+    pkgs.pkg-config
+    pkgs.cmake
+  ];
+
+  # Narrowed source tree the rust derivations consume.
+  # `cleanCargoSource` is crane's standard "everything cargo cares
+  # about" filter (Cargo.toml/Cargo.lock + *.rs). All non-rust runtime
+  # assets — branding + the claude system prompt template +
+  # claude-settings.json — live in the separate `hyperhive-assets`
+  # derivation and are loaded by the binaries at runtime from
+  # `$HIVE_ASSETS_DIR`. The unit tests in `hive-ag3nt::prompt` read
+  # `prompts/system.md` directly from the workspace tree at *test*
+  # runtime (via `env!("CARGO_MANIFEST_DIR")` — a compile-time string,
+  # no file open at compile), so the prompt template doesn't have to
+  # be in this fileset to keep `cargo test` honest. Net effect: tweaks
+  # to any non-`*.rs` / non-`Cargo.*` file (README, branding, nix
+  # modules, frontend tree, OR `hive-ag3nt/prompts/*`) do NOT bust
+  # this src hash, so the rust derivations stay fully cached.
+  cleanSrc = craneLib.cleanCargoSource ../.;
+
+  # Build the workspace's dependency tree once, cached as its own
+  # derivation. Package builds and checks reuse this via
+  # `inherit cargoArtifacts;` so a workspace-only edit doesn't rebuild
+  # deps. All consumers use the same `cleanSrc` so the input hash
+  # stays consistent across the chain.
+  cargoArtifacts = craneLib.buildDepsOnly {
+    src = cleanSrc;
+    # Workspace Cargo.toml is virtual (no `[package].name`), so crane
+    # can't auto-derive a name. Spell it out explicitly — keeps the
+    # derivation name stable across crane bumps + silences the
+    # placeholder warning. Same `pname` for the dep + check
+    # derivations so they share a clean naming family.
+    pname = "hyperhive-workspace";
+    version = "0.1.0";
+    inherit nativeBuildInputs;
+  };
+}