The tab keeps the last state error shown and follows the error at the cursor,
steps through several errors, counts them on the tab, and links back to the
error in the Verification tab. State errors get a labelled Show in state
machine button and CodeLens.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Count only the state errors of the current file, keep the focus toggle with
the legend apart from View error, and move Show in state machine to the top
of the error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

## Description
Stacked on #159; merge that first. Independent of #161.

The tabs are now **Verification · State Machine · Context**.
Verification and State Machine link to each other through **Show in
state machine** and **View error**, so they now sit side by side.
Context shows the variables at the cursor for any file and works fine at
the end.

The walkthrough steps and the README sentence that lists them follow the
same order.

## Screenshots
`E_OSS_A1_19_AccessKeyGenerator` from the C-stars proposal in
building-examples-liquidjava.

**Before**

<img width="560" alt="Tabs: Verification, Context, State Machine"
src="https://raw.githubusercontent.com/liquid-java/vscode-liquidjava/pr-screenshots/tab-order/before.png"
/>

**After**

<img width="560" alt="Tabs: Verification, State Machine, Context"
src="https://raw.githubusercontent.com/liquid-java/vscode-liquidjava/pr-screenshots/tab-order/after.png"
/>

## Tests
- `npm run lint` and `npm run test:typecheck`: clean.
- `npm run test:unit`: 90 passed. New test in `script.spec.ts` for the
tab order.
- `npm test` (VS Code): passed; the walkthrough test now expects the new
step order.
- Server `mvn test` and the local smoke test: passed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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