feat: add the pure data-safety planner

Phase 3 shipped conflict and move sequencing but skipped the data-safety checks the design spec makes the pure planner's job. This adds them: same-file rejection, dir-into-itself rejection, trailing-slash destination resolution, special-file rejection, symlink surfacing, case-insensitive collision detection, and path-length and reserved-name checks.

Every check is pure. The filesystem facts each one needs (a path's lstat type, the names already at a destination, whether that filesystem folds case, its path-length limit, its reserved names) are injected, so the planner decides before a byte moves and the tests never touch a real file. Each problem is a plist with a class, a severity, the file, and a message. An error severity blocks a transfer, and a warning surfaces a decision, which is how a symlink is carried as follow-versus-preserve rather than silently chosen.

duet--plan-move-safe composes the checks with the move planner: a source carrying a blocking error is skipped with no copy and no delete, a safe source gets a copy and a delete gated on its copy success, and no delete is ever ungated. Coverage on duet.el rises to 92.3%.

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