test: verify jump-to-slide shortcut works correctly with countIncrementalSlides=false

[dpecos]

Summary

• Adds regression tests for the scenario described in issue Bug: jump-to-slide shortcut uses wrong numbers when countIncrementalSlides is false #10
• Confirms that byNumber mapping already handles countIncrementalSlides: false correctly
• Typing a displayed slide number navigates to the expected slide

Investigation

After tracing through createSlides() in slideshow.ts, the bug from issue #10 could not be reproduced. The byNumber mapping correctly:

• Assigns slide numbers [1, 2, 2, 3] when an incremental (--) slide is present and countIncrementalSlides: false
• Groups Slide 2 and its incremental Slide 2b under byNumber[2]
• Maps byNumber[3] exclusively to Slide 3 (the last slide)
• Ensures typing 3 + Enter navigates to Slide 3 as expected

Tests added

Six new assertions under number mapping > jump-to-slide with countIncrementalSlides: false (issue #10) covering the exact scenario from the issue report.

Closes #10