Render content images at their original position in the prose flow

[yrstm]

Why

Mantis previously appended all images as a batch at the end of Markdown while data tables were already rendered inline. Content images should stay near the prose and tables they accompanied on the page.

What

• Live-DOM content images now carry a position anchor: the index of the captured block they follow, or -1 when they lead the document.
• Images and data tables are positioned together and receive a shared flowOrder, preserving their actual DOM order when they share an anchor.
• toMarkdown() validates anchors before splicing. Missing, fractional, non-finite, negative out-of-range, and past-the-end positions use the trailing fallback instead of being dropped.
• Positioned images retain render priority 4, so the outline budget still sheds images first and images do not consume a section's lead priority.
• unpositionedImages, MantisImage.position, and the shared flowOrder fields are documented in the type declarations.
• README and public-page changelog copy describe the behavior.

Compatibility

• Stored and vision-pipeline articles without positions keep the previous trailing-list behavior.
• No image downloading is introduced; Markdown continues to reference remote URLs.
• Table rendering and the clipboard-first flow are unchanged.

Testing

• 7 focused tests cover inline image placement, leading/title-adjacent images, mixed image/table DOM order in both directions, invalid-anchor fallback, stored-article fallback, and transient DOM-reference cleanup.
• npm test: 70 passing.
• npm run types:check: passing.
• npm run benchmark: 129/129.
• npm run perf: fidelity checks passing.
• npm pack --dry-run: passing.
• GitHub Actions test and demo-browser: passing.