simplicity: add 'simplicity pset extract' command

Now rather than extracting using elements-cli, by running

    elements-cli finalizepsbt <psbt> true

you can do it with hal-simplicity, with

    hal-simplicity simplicity pset extract <psbt>

Author: apoelstra

src/bin/hal-simplicity/cmd/simplicity/pset/extract.rs

@@ -0,0 +1,29 @@
+// Copyright 2025 Andrew Poelstra
+// SPDX-License-Identifier: CC0-1.0
+
+use elements::encode::serialize_hex;
+
+use super::super::{Error, ErrorExt as _};
+use crate::cmd;
+
+pub fn cmd<'a>() -> clap::App<'a, 'a> {
+	cmd::subcommand("extract", "extract a raw transaction from a completed PSET")
+		.args(&cmd::opts_networks())
+		.args(&[cmd::arg("pset", "PSET to update (base64)").takes_value(true).required(true)])
+}
+
+pub fn exec<'a>(matches: &clap::ArgMatches<'a>) {
+	let pset_b64 = matches.value_of("pset").expect("tx mandatory");
+	match exec_inner(pset_b64) {
+		Ok(info) => cmd::print_output(matches, &info),
+		Err(e) => cmd::print_output(matches, &e),
+	}
+}
+
+fn exec_inner(pset_b64: &str) -> Result<String, Error> {
+	let pset: elements::pset::PartiallySignedTransaction =
+		pset_b64.parse().result_context("decoding PSET")?;
+
+	let tx = pset.extract_tx().result_context("extracting transaction")?;
+	Ok(serialize_hex(&tx))
+}

src/bin/hal-simplicity/cmd/simplicity/pset/mod.rs

@@ -1,6 +1,7 @@
 // Copyright 2025 Andrew Poelstra
 // SPDX-License-Identifier: CC0-1.0
 
+mod extract;
 mod finalize;
 mod update_input;
 
@@ -15,12 +16,14 @@ struct UpdatedPset {
 
 pub fn cmd<'a>() -> clap::App<'a, 'a> {
 	cmd::subcommand_group("pset", "manipulate PSETs for spending from Simplicity programs")
+		.subcommand(self::extract::cmd())
 		.subcommand(self::finalize::cmd())
 		.subcommand(self::update_input::cmd())
 }
 
 pub fn exec<'a>(matches: &clap::ArgMatches<'a>) {
 	match matches.subcommand() {
+		("extract", Some(m)) => self::extract::exec(m),
 		("finalize", Some(m)) => self::finalize::exec(m),
 		("update-input", Some(m)) => self::update_input::exec(m),
 		(_, _) => unreachable!("clap prints help"),