Generalize source projection

[Azganoth]

Summary

• Rename the inline source-projection capability and related modules to sourceProjection.
• Generalize projection sessions around immutable ProseMirror slices and object-specific adapters.
• Move target discovery, edit policy, source parsing, presentation spans, selection mapping, and restoration behind the adapter contract.
• Preserve exact clean restoration, literal invalid-source fallback, local history, native undo/redo, dirty state, and serialization behavior.
• Add non-mark contract coverage for atomic-node selection, editing, restoration, rehydration, and target switching.
• Restore projected inline-code, strikethrough, and link visual parity.

Related Issue

Closes #63

Notes

This is an enabling rework only. Mixed-format link-label projection remains in #58, and footnote-reference projection remains in #60; neither follow-up depends on the other.