build: static musl release build capability

rust-toolchain.toml gains the x86_64-unknown-linux-musl target; the
flake devShell gains a musl cc (pkgsCross.musl64) + CC/linker env so a
`cargo build --target …-musl` compiles rusqlite's bundled SQLite C and
links fully static (D2: single static binary, no runtime deps). Cargo
release profile strips symbols (13MB -> 10MB). Verified locally: the
musl binary is static-pie, statically linked, stripped, runs standalone.

flake.nix

@@ -27,6 +27,13 @@
         # from the crate metadata (no hand-maintained duplicate here).
         cargoToml = builtins.fromTOML (builtins.readFile ./Cargo.toml);
 
+        # musl-targeting C compiler for the static release build: rusqlite's
+        # `bundled` SQLite is compiled from C, so a `--target …-musl` cargo build
+        # needs a musl `cc` for that C, plus a musl linker. `targetPrefix` is
+        # "x86_64-unknown-linux-musl-", so the wrapped binary is
+        # `${muslCC}/bin/x86_64-unknown-linux-musl-cc`.
+        muslCC = pkgs.pkgsCross.musl64.stdenv.cc;
+
         # System build inputs are deliberately tiny — this is a pure-Rust TUI:
         #   * libsqlite3-sys is built with the `bundled` feature, so SQLite is
         #     compiled from vendored C. That needs a C compiler, which the
@@ -68,9 +75,17 @@
             # CLAUDE.md "Build hygiene"). cargo-sweep prunes them; run it
             # periodically between milestones.
             pkgs.cargo-sweep
+            # musl cc/linker for the static release build (see muslCC above).
+            muslCC
           ];
 
+          # Point cargo's musl target at the musl cc for both the bundled-C
+          # compile (CC_<target>, consumed by the `cc` crate) and the final link
+          # (CARGO_TARGET_<TARGET>_LINKER). Harmless for normal glibc builds —
+          # these only take effect when building `--target x86_64-…-musl`.
           shellHook = ''
+            export CC_x86_64_unknown_linux_musl="${muslCC}/bin/${muslCC.targetPrefix}cc"
+            export CARGO_TARGET_X86_64_UNKNOWN_LINUX_MUSL_LINKER="${muslCC}/bin/${muslCC.targetPrefix}cc"
             echo "RDBMS Playground dev shell ($(uname -s))"
             echo "  rust:  $(rustc --version | cut -d' ' -f1-2)"
             echo "  cargo: $(cargo --version | cut -d' ' -f1-2)"