feat(ui): implement Map grouping, dependents, and weight bars; symbol tab address

Adds a new grouping system for the Map with a new grouping depth control, exposes per-module dependents (files and modules) to drive a weight bar, and renders it on each module. Introduces a MapKey to explain visuals, collapses lone root-file buckets for clearer labeling, and supports a nullable depth value to let the provider pick grouping. The Symbol tab now has its own address (#/s) when nothing is selected, and routing/top-bar logic is updated accordingly. Also updates export SVG rendering to include weight-based bars, and extends tests and docs to cover the new visuals and behavior.

CHANGELOG.md

@@ -201,6 +201,10 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 #### Symbols, tests and the viewer
 
+- **The Map groups a repository the way that repository is shaped.** It always drew top-level directories, so a project whose whole program lives under one `src/` opened as a picture of four boxes — `src`, `ios`, `.github`, `(root files)` — with two thirds of the code inside one of them and nothing to say about it. The Map now picks its own grouping: the shallowest one that is not a single box holding the program, so a mobile app opens on `src/app`, `src/components`, `src/api`, `ios/CaptureView` and the rest, and a project packaged as `frontend/src/…` opens on the screens, components and reducers instead of on the word `frontend`. A repository whose top-level directories really are its modules is left exactly where it was. A new **Grouping** control on the right says which one was chosen and lets you take it a level in or out, and a leaf directory is now named for itself rather than as `…/(root files)`. Each box now also says how much leans on it — how many files elsewhere reference straight into it — with a bar along its bottom edge scaled against the most depended-on box on screen, so the folder you have to be careful with is the one you can see at a glance rather than the one with the longest name. The Map also has a **Key** now, like the Screens and Steps tabs — including what the dashed maroon lines mean, which only appear once you select a module: that module reaching back UP into something that depends on it.
+
+- **The Symbol tab opens the Symbol tab.** With no symbol open and no trail to return to, clicking **Symbol** in the top bar took you to the landing page — which, on any project that has screens, is the Screens tab. So the button said Symbol and gave you somebody else's view. It now has an address of its own (`#/s`) that opens the "nothing selected" screen: the search prompt and the where-to-start list of routes, entry files and the symbols the most code depends on.
+
 - **Files under an `e2e/` directory count as tests.** Their calls no longer appear as production callers in Steps, dead-code and test badges.
 
 - **Production code under a `samples` or `examples` package path is no longer treated as test code.** A Kotlin or Java project whose package path runs through `com/google/samples/…` (Now in Android, for one) had nearly every file counted as a fixture, so the Map opened on `build-logic`, the entry points hid the app, and dead-code and test badges were wrong. Only the project layout above a `src/` folder decides now; the package path below it never does.