Feature(launch): let --assets-path add assets roots to the core one

[cmoron]

Closes #46.

What

--assets-path now adds an assets root instead of replacing the core one, so a
project that consumes LOTUSim can keep its own worlds and models in its own
repository and still run them through lotusim run:

lotusim --assets-path /my/scenario/assets run my_scenario.world

The core catalogue stays available — the sailboat from assets/models and the buoy
from the scenario resolve in the same world.

Details

• The core root is always searched first; --assets-path appends after it.
• The option accepts a colon-separated list (--assets-path "$A:$B") and is
  repeatable (--assets-path $A --assets-path $B). Both forms are equivalent — the
  colon form mirrors the GZ_SIM_RESOURCE_PATH this feeds.
• Worlds are resolved from whichever root holds them.
• A missing world now names every root searched, instead of being handed to gz and
  failing there with a less obvious message.
• Shell completion follows the same roots, so worlds outside the core are offered.

Behaviour change, stated plainly

A single --assets-path used to replace the core root; it now adds to it.

We looked for callers before changing this: the option appears nowhere in this
repository, its CI, its documentation, the wiki, or LOTUSim-generic-scenario — only
in the completion script's option list. The replacing semantics were also the direct
cause of the limitation in #46. If you would rather keep them untouched, we are happy
to move the additive behaviour behind a separate flag.

There is no longer a way to exclude the core root. Nobody asked for that isolation and
an extra search entry is harmless, so we did not invent a flag for it.

Verification

launch/tests/test_assets_path.sh is self-contained: it builds two throwaway assets
roots under mktemp -d, touches nothing in the workspace, and needs only a built
LOTUSim environment.

bash launch/tests/test_assets_path.sh
===== 9 passed / 0 failed =====

It covers the unchanged default, the additive single option, both multi-root forms and
their equivalence, world resolution from the owning root, the error message, and
completion. The decisive case is an A/B on gz itself: a world in root B that includes
model://alpha from root A loads with both roots and fails with
Unable to find uri[model://alpha] without root A — so the test would notice if the
composition silently stopped working.

Note for anyone reproducing it: the check needs --debug, because at the default
-v0 gz reports nothing and the A/B would prove nothing.

Run against ghcr.io/naval-group/lotusim (Jazzy / Harmonic, noble).

Not included

The wiki lives in its own repository, so the --assets-path entry in Getting-Started
is not part of this PR. Happy to send that separately if useful.

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Assisted-by: Claude Opus 5 (claude-opus-5) via Claude Code.