feat(viewer): surface the project facet in the table and stats

[njordr]

Summary

project is already a first-class field everywhere except the screen. It is parsed into Frontmatter, exposed through Adr::project(), offered in the viewer's Project filter dropdown, and counted in AdrStatistics::by_project with test coverage in test_statistics_by_project.

Nothing ever renders it. A collection grouped by project can be filtered by project, but the value itself is invisible in the table, in the detail panel, and in stats output. Selecting a project in the dropdown narrows the list, and the reader still cannot tell which project any row belongs to.

This surfaces the facet through the paths that already exist for category and author.

Changes

• List view: a sortable Project column, rendered as plain text next to Author. Plain rather than a badge, so it does not compete with the category chip.
• Detail panel: a Project row beside Category.
• Stats: By Project in the summary and in the Markdown output, mirroring the category and author breakdowns.
• Layout: overflow-x: auto on .view-container, so the seventh column scrolls instead of pushing the table past a narrow viewport.

Notes

• No new frontmatter, no schema change, no new dependency. Records without a project show -, exactly as they already do for a missing author.
• test_summary_with_all_fields now sets a project on both fixtures and asserts By Project: alongside the existing By Category: and Authors: assertions.
• make check passes: cargo fmt --check, cargo clippy -- -D warnings, and the full test suite.