v0.57.1: media polish and cutover DX

Media polish and cutover DX, the first follow-on after the 0.57.0 media stack.

• Media Library action feedback. A delete, a rename, and a commit conflict now land on a strip that confirms the result or shows the error, instead of a silent page.
• Slide-over Escape. With the detail panel open and focus in the search box, Escape clears the search and leaves the panel open, rather than doing both at once.
• Decorative hero alt. A frontmatter hero marked decorative persists that choice as an additive decorative key on the image object, so a deliberately decorative hero stops reading as needs-alt after a reload. A decorative body image still cannot persist the choice, since markdown alt text has no slot for it.
• Clearer reserved-figure error. The build error now names the colliding component and points at the fix (remove a custom figure the engine's built-in now covers, or rename it).
• Media docs. The public media resolver wiring moved into the required media setup steps, since a published media: token ships bare without it. The reserved-figure collision is now a prominent breaking callout. A new content authoring syntax reference documents the cairn: and media: token schemes, and the guides gain the wrangler.toml binding dialect, the @glw907/cairn-cms/media import path, the empty-media.json bootstrap, and the figure-placement CSS re-scope.

No consumer action required. The decorative key is additive and optional, so existing content parses and builds unchanged, and the feedback strip, the Escape fix, and the registry error message are admin or build-time with no public surface change.

Full notes in CHANGELOG.md (0.57.1).