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.

Cargo.toml

@@ -68,6 +68,12 @@ tempfile = "3.27.0"
 incremental = false
 debug = "line-tables-only"
 
+# Release builds back the distributed binaries (D2: single static binary).
+# strip = "symbols" drops the symbol table at link time so the shipped artifact
+# is lean (≈13 MB → 10 MB for the musl build) without a separate strip step.
+[profile.release]
+strip = "symbols"
+
 [lints.rust]
 unsafe_code = "forbid"
 unreachable_pub = "warn"