fix(ui-map): count depth in folders that fork, so a Maven project opens on its packages (#2102)

A Maven / Gradle project keeps every line of Java under
src/main/java/org/<company>/<app>/, and those folders hold nothing but the
next one. The Map cut one folder per level, so petclinic drew the whole
program as one `src/main/java/org` box, and the deepest grouping option
(4) stopped at `src/main/java/org/springframework` — no setting reached the
packages seven levels down.

A folder with exactly one subfolder and no file of its own (read from
every indexed file, tests included, so ids don't move when tests are
toggled) now joins the level below it. moduleIdFor takes the set as an
optional fourth argument — without it nothing changes — and
pickDefaultDepth counts the same levels. Collapsing never splits a group
at the same depth (a pass-through folder has one child), so repos without
such chains keep their grouping; a lone chain like koel's
`app/Console` → `app/Console/Commands` is renamed to the folder that holds
the files.

The box label (the wire `label`, which nothing read) now elides a chain of
three or more folders — `src/main/java/…/petclinic/owner` — and ModuleNode
and the width calculation use it; the tooltip and side panel keep the full
path. Everywhere else the label equals the id, so other maps look the same.

petclinic's default map: 8 boxes (owner, vet, model, system, the package's
root files, resources) instead of one; realworld opens on io/spring's api,
application, core, graphql, infrastructure.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/viewer-launch-changelog.md

@@ -161,6 +161,8 @@ These describe `codegraph ui` and its screens. They were taken out of `## [Unrel
 
 ## Fixes — Symbols, tests and the viewer
 
+- **The Map opens a Maven or Gradle project on its packages.** A Java project keeps every file under `src/main/java/org/<company>/<app>/`, and those folders hold nothing but the next one, so the Map drew the whole program as one `src/main/java/org` box and no grouping option reached further. A folder with one subfolder and no files of its own no longer counts as a level: petclinic opens on `owner`, `vet`, `model` and `system`, each labelled `src/main/java/…/petclinic/owner`, with the full path on hover.
+
 - In `codegraph ui`, routes whose handlers live in more than 60 different files are all linked to their handler, instead of the later ones showing "not in the index". (#1975)
 
 - The viewer continues to count module-level initializer calls as top-level file activity in entry points and file screens.