Document image support; add Images demo and snapshot test - #146

Finalize block-level image support (parts 1-4 already shipped) with the
non-rendering deliverables:

- README: move Images into the supported list with an accurate description
  of the experimental ImageConfig source types and fullscreen viewer.
- Sample app: add an Images demonstration (enum case, subtitle, fixture)
  showcasing asset-catalog, bundled-resource, and remote sources, plus
  image-alongside-text splitting and an unresolved-source placeholder.
- Tests: add ImageBlockSnapshotTests rendering a deterministic bundled
  image via Image(mdImage:); bundle test resources via Package.swift and
  record the iOS reference snapshots.

Refs #89

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 17e2748f-66c1-4deb-b557-df65a17c84f5

Addresses review comments on the image-support PR:

- Add `MarkdownListener.resolveBundledResource(fileName:ext:)` (default
  returns nil). The renderer now resolves bundled-resource images from the
  app's main bundle first and falls back to the listener when absent, so
  consumers can serve images that live in a dependency package or framework
  bundle rather than the app target. Threaded through BundledResourceImage,
  the fullscreen viewer, and the image-tap payload.
- README: move Images up under Paragraphs in the supported list.
- Tests: parse Markdown containing a bundled image and verify end-to-end
  resolution via the listener (the resource lives only in the test bundle);
  cover the no-listener case; render the image deterministically via
  Image(mdImage:) for the visual snapshot. Replace the helper's fatalError
  with throwing XCTUnwrap so a missing resource is a normal test failure.

Refs #89

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 17e2748f-66c1-4deb-b557-df65a17c84f5

Addresses the second review round on PR #146:

- Replace the bundled-image snapshot with the deterministic failure case: a
  remote image whose host is not in the allowed domains resolves to nil and
  renders BlockImageFailureView synchronously. The remote and bundled loading
  paths render asynchronously via `.task`, which the synchronous snapshot
  harness captures as the loading placeholder, so they can't be snapshotted
  deterministically.
- Remove the direct Image(mdImage:) snapshot and the bundled-resource
  pipeline-logic tests from the snapshot file; move listener-fallback
  resolution coverage into ImageTapTests as a fast unit test.
- Make ImageData.makeMarkdownImage(controller:) take a non-optional
  MarkdownController (the default only existed for test convenience); unwrap
  the environment controller in BlockImageView before building the payload.

Refs #89

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 17e2748f-66c1-4deb-b557-df65a17c84f5

Addresses the latest review comments on PR #146:

- Snapshot: the failure-placeholder test now parses a paragraph before and
  after the image, so the rendered DocumentView shows text/image/text.
- Revert makeMarkdownImage(controller:) to an optional MarkdownController.
  SwiftUI's environment injection does not guarantee a non-nil controller, so
  BlockImageView optionally chains onImageTap rather than guarding out.

Refs #89

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 17e2748f-66c1-4deb-b557-df65a17c84f5