Support symlink and submodule changes in structural diffs #67

Support symlink-target and submodule commit-pointer changes in structural diffs instead of returning unsupported_file_type.

Today these file errors cause consumers such as Whiteboard to fail progress calculation unless they explicitly skip unsupported entries. Whiteboard currently omits them; it should be able to consume native diffr results once support lands.

• Represent added, modified, and deleted symlink targets without dereferencing the links.
• Represent added, modified, and deleted submodule commit pointers without requiring a populated submodule checkout. Recursive submodule content diffs are out of scope.
• Define appropriate rendering and coverage/count semantics for these entries.
• Cover tree comparisons and working-tree comparisons, including file-type transitions, and ensure these entries do not produce failed-file outcomes.
• Update Whiteboard's bundled diffr and remove its skip behavior once supported.

Context: devdotfast/whiteboard#829
Temporary consumer handling: devdotfast/whiteboard#833