Generalize and rename the source-projection session engine

[Azganoth]

Summary

Generalize Leafdown’s existing inline source-projection plugin into a reusable source-projection session engine and rename the capability from inlineSourceProjection to sourceProjection.

This is a behavior-preserving enabling rework for:

• Mixed-format link labels do not enter source projection #58 — mixed-format link labels;
• Project footnote-reference source in the editor #60 — atomic footnote-reference nodes.

The current session representation assumes one text range with a uniform set of marks and reconstructs canonical content from text + marks. It cannot safely represent either a rich inline fragment or an inline atomic node.

Retain the established session lifecycle: projection-local undo/redo, restore-before-commit native history bridging, dirty-state integration, transient Markdown-update suppression, selection preservation, target switching, save-time finalization, and exact literal fallback.

Expected behavior

• The shared engine owns one active session, its projected range, original ProseMirror content, projected source, local history, transaction metadata, and finalization lifecycle.
• Sessions preserve the original target as an immutable ProseMirror Slice or equivalent fragment rather than reconstructing it only from uniform marks.
• Projection entry replaces the complete target with unmarked editable source text.
• Clean finalization restores the original content exactly.
• Edited finalization restores the original content before applying the adapter-produced replacement as the native history change.
• Invalid source commits every projected character as literal document text.
• Object-specific adapters own target discovery and precedence, source generation, validation and rehydration, presentation spans, and selection mapping.
• The adapter contract supports rich inline fragments and inline atomic nodes without implementing Mixed-format link labels do not enter source projection #58 or Project footnote-reference source in the editor #60 in this issue.
• Existing strong, emphasis, strikethrough, inline-code, link, and autolink projection behavior remains unchanged.
• The plugin, syntax modules, exported symbols, tests, imports, CSS selectors, data attributes, and documentation use sourceProjection terminology.
• markerPresentation remains a separate capability.
• The contract allows Mixed-format link labels do not enter source projection #58 to preserve one logical outer link wrapper when a mixed-format link is saved and reopened.

Related docs

• Specification: Marker visibility and presentation
• Specification: Inline content
• Decisions: Treat marker presentation as object-specific
• Inline source projection spike
• Design seamless inline Markdown source projection #44
• Implement inline source projection for strong and emphasis #45
• Mixed-format link labels do not enter source projection #58
• Project footnote-reference source in the editor #60

Done when

• The capability consistently uses sourceProjection terminology.
• Sessions store and restore original ProseMirror slices or fragments.
• Session lifecycle logic is independent of mark-specific syntax.
• An adapter boundary exists for discovery, source generation, rehydration, presentation, and selection mapping.
• Clean entry and exit restore original document content exactly.
• Edited valid source retains existing native undo/redo behavior.
• Invalid source retains every projected character as literal text.
• Existing projection behavior remains unchanged and covered.
• Projection-local history, dirty tracking, Markdown-update suppression, selection crossing, target switching, paste, and save-time finalization remain covered.
• Projection documentation and the documentation index use the new capability name.
• Mixed-format link labels do not enter source projection #58 and Project footnote-reference source in the editor #60 can depend on this issue without depending on each other.

Out of scope

• Mixed-format link-label projection itself.
• The serializer behavior required to preserve one logical link wrapper.
• Footnote-reference projection.
• Footnote definitions.
• Raw HTML, images, and tooltip UI.
• New user-visible source-projection behavior.
• A permanent Markdown-token schema.