feat(report): add offline observability deck shell

[Trecek]

Adds autoskillit sessions deck <output> to render the derived report index as a self-contained observability deck that opens from file:// without a server or network requests.

The shell includes question-based navigation, a pinned cohort bar with live/struck/absent chip states and visible reasons, hash routing that preserves filters, sortable tables, SVG bars, light/dark palettes, and print styles. “Who is in this cohort?” is the built view; analytical views remain planned entries with their enabling issues. The HTML assembler embeds escaped JSON and hashes each executable script for CSP.

Closes #4650.

Validation:

• Each implementation phase passed the configured test_check gate; final result: 50,683 passed, 614 skipped, 29 xfailed.
• pre-commit run --all-files passed, including formatting, typing, lockfile and architecture checks.
• Offline Chromium checks passed against a deck with 2,000 real session rows: zero network requests or page errors, all chip states, filters, history navigation, identifier links, sorting, accessible chart labels/values, both palettes, and print styles.
• The chart-only PDF contains no raster image objects. Chromium rasterizes some CSS hatch patterns in the full-page PDF; the bars remain vector.