Docs for the Rust version; benchmarks; build.sh fixes

README, CLAUDE.md, man pages, the user guide and its screenshot script
now describe the Rust build and the intentional behaviour changes, which
docs/MIGRATION.md lists. Go and Rust micro-benchmarks for the same
operations, and scripts/bench.sh for end-to-end comparison. build.sh
exports its tree root to the per-target jobs and keeps logs out of the
output directory; riscv64 musl builds use getrandom's raw syscall
backend, since zig's musl getrandom never reaches the kernel there.

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.toml

@@ -40,3 +40,7 @@ windows-sys = { version = "0.61", features = ["Win32_Foundation", "Win32_Storage
 codegen-units = 1
 lto = true
 strip = true
+
+[[bench]]
+name = "micro"
+harness = false