feat(canopy-typst): agency seal / logo SVG assets for letterhead.typ

Source: typst-document-generation plan Potential Improvements.

rulesets/georgia/notices/assets/ is empty. The Georgia DHS seal/logo SVG should be added and integrated into letterhead.typ so generated notices render with the correct agency identity.

Acceptance: Georgia DHS logo SVG committed; letterhead.typ references the file; visual snapshot test confirms correct top-of-page rendering.