emit build-database rewrites the build program's host mcpp.lock on every run and lists it in data.watch, so a watcher re-runs emit forever #807

Summary

mcpp emit build-database --format json lists a file in data.watch that the same command rewrites on every run. The file is the mcpp.lock of the build program's synthesized host package:

~/.mcpp/cache/build-database/<h>/target/.build-mcpp/host/<h>/mcpp.lock

The content stays the same; only the mtime changes. A tool that watches data.watch and runs emit again when a watched file changes never settles: each emit "changes" the file that triggers the next one.

Seen in mcpp-language-server 0.0.15 on a project with a build.mcpp and [build-dependencies] (a Qt Widgets demo using mcpp:plugins rules-qt). It runs mcpp emit every 2 seconds for as long as the project is open. The log repeats project model loaded again: unchanged, each run preceded by a didChangeWatchedFiles for this mcpp.lock (type 2, changed).

Reproduce (mcpp 2026.10.11.1, Linux x86_64)

Any project whose build.mcpp has build dependencies, for example:

[build-dependencies.mcpp]
plugins = { version = "0.15.2", features = ["rules-qt"], host-module = true }

mcpp emit build-database --format json > /tmp/emit.json
python3 -c "import json; print([w for w in json.load(open('/tmp/emit.json'))['data']['watch'] if w.endswith('mcpp.lock')])"
# ['…/.mcpp/cache/build-database/5042b60d2f765ba8/target/.build-mcpp/host/e50cbbe8641b/mcpp.lock', 'mcpp.lock']

f=~/.mcpp/cache/build-database/5042b60d2f765ba8/target/.build-mcpp/host/e50cbbe8641b/mcpp.lock
stat -c %Y "$f"; md5sum < "$f"     # 1791919945  0c27208d1dd09132234081d90d03aa3e
sleep 1; mcpp emit build-database --format json > /dev/null
stat -c %Y "$f"; md5sum < "$f"     # 1791919946  0c27208d1dd09132234081d90d03aa3e   <- rewritten, same bytes

Where

• src/build/prepare/records.cpp:314: (void)mcpp::lockfile::write(lock, lockPath); runs unconditionally, so a resolution that did not change still rewrites the lock.
• src/build/build_database.cppm:591: watch_file(ctx.projectRoot / "mcpp.lock");. For the build-program member, projectRoot is the synthesized host package under the build-database cache, so a file mcpp itself writes is reported as an input.

Suggested fix

1. Write the lock only when its bytes differ from what is on disk (write-if-changed). This also helps anything else that watches a project's own mcpp.lock: editors, cargo watch-style tools, file sync.
2. Optionally, leave the synthesized host package's mcpp.lock out of data.watch. It is an output of the plan, not something a person edits. The inputs that decide it (the root mcpp.toml, build.mcpp, the root mcpp.lock) are already listed.

mcpp-language-server will also stop reacting to a watched file whose content did not change, so it no longer depends on this. Other consumers of data.watch still would.