Document session and save path !5

Adds the OrgDocument target and its plan.

• threeWayMerge and lineEdits: line-based, Myers diff with prefix/suffix trimming, line endings kept.
• DocumentState: edits against a revision through incremental reparse, grouped undo/redo, BOM-preserving encoding, reload of an unedited buffer, merge into an edited one, conflicts leave the buffer alone.
• ViewState: selection and folds mapped through edits; folds pruned to existing headings.
• Saver: read, merge if the file moved, check again, replace, read back. Every displaced version ends up on disk, in the buffer, or in recovery.
• CoordinatedFileSystem (NSFileCoordinator, temp file, replaceItemAt with a kept backup) and FileRecoveryStore (last 20 per file).
• Fault-injection tests put another writer's change before each read and before the replace. 30 new tests; all 117 pass; OrgDocument builds for iOS.