G2: --save runs on one project take turns, so no edit is lost (#2851)

A run reads the project, edits it and writes the whole file back, so two
at once silently dropped the edits of whichever saved first. A run that
saves over the file it opened (--save, or --save-as the same file) now
holds an OS lock on `.<name>.lock` next to the project from before its
read until it exits. The next run says so on stderr, waits, then edits
what the first one saved. Ported from filmcraft-cli
(storytold/filmcraft#3926).

The OS drops the lock however the process ends, so a crash leaves nothing
stale. If the lock can't be taken the run warns and saves as it did
before. A missing project stops the run before it opens anything, with
the same error opening gives, so a project created in between is never
saved without the lock. The desktop app and `mcp` don't take this lock.

apps/effectcraft-cli/src/main.rs

@@ -47,6 +47,7 @@
 
 use std::io::Write;
 
+mod project_lock;
 mod template;
 
 use effectcraft_automation::tools::{self, Reply};
@@ -508,6 +509,7 @@ fn script_cmd(args: &Args, json_out: bool) -> Result<(), Failure> {
         (None, Some(f)) => (std::fs::read_to_string(f).map_err(|e| Failure::Error(format!("cannot read {f}: {e}")))?, f.clone()),
         (None, None) => return usage_err("script needs a .jsx/.js file or --eval CODE"),
     };
+    let _lock = project_lock::for_save(args)?;
     let mut b = backend(args, false)?;
     let r = b.exec("script.run", json!({"code": code, "name": name}))?;
     let saved = maybe_save(&mut b, args)?;
@@ -611,6 +613,7 @@ fn run(cmd: &str, args: &Args, json_out: bool) -> Result<(), Failure> {
                 (Some(p), _) | (None, Some(p)) => serde_json::from_str(p).map_err(|e| Failure::Usage(format!("params are not valid JSON: {e}")))?,
                 (None, None) => json!({}),
             };
+            let _lock = project_lock::for_save(args)?;
             let mut b = backend(args, true)?;
             let r = b.exec(&id, params)?;
             if let Some(e) = failed_render(&id, &r) {
@@ -654,6 +657,7 @@ fn run(cmd: &str, args: &Args, json_out: bool) -> Result<(), Failure> {
             if steps.is_empty() {
                 return Err(Failure::Usage(format!("--commands {}: no commands", sources.join(", "))));
             }
+            let _lock = project_lock::for_save(args)?;
             let mut b = backend(args, true)?;
             let mut results = vec![];
             // The step's index closes the message: a generated list repeats ids, and callers match
@@ -708,6 +712,7 @@ fn run(cmd: &str, args: &Args, json_out: bool) -> Result<(), Failure> {
             }
             let (comp, layer, path) = (args.pos[0].clone(), args.pos[1].clone(), args.pos[2].clone());
             let value = args.pos.get(3).map(|v| value_arg(v));
+            let _lock = project_lock::for_save(args)?;
             let mut b = backend(args, true)?;
             let a =
                 json!({"comp": reference(&comp), "layer": reference(&layer), "path": path, "value": value, "time": args.num("--time")?, "expression": expr});