`sysand add urn:kpar:sysmod` broken

[consideRatio]

Testing #183 i observed that access of index was already broken in main, where I test things like this:

d=$(mktemp -d) && cd "$d" \
  && cargo run --manifest-path ~/dev/sensmetry/sysand/Cargo.toml \
    init \
  && cargo run --manifest-path ~/dev/sensmetry/sysand/Cargo.toml \
    add urn:kpar:sysmod

The failure stems from #130, verified by testing before and after.

I used codex (GPT 5.4) to debug and review the situation, and it came up with some changes #205.

[consideRatio]

Ping @andrius-puksta-sensmetry and @victor-linroth-sensmetry for awareness

[victor-linroth-sensmetry]

Simplest solution is just to have InMemoryProject return it's nominal_sources and remove the debug_assert!(!sources.is_empty()) from do_lock_extend. Having the standard library not return sources (unless using --include-std) seems pretty baked in at the moment and probably requires more though if we want to change that.

[andrius-puksta-sensmetry]

In my opinion it would be better to not even try including sources for provided_iris, which would avoid this problem.