Harden scanner accounting and expose controlled scan reports

MANUAL.md

@@ -35,9 +35,24 @@ sized [path]
 ```
 
 ### Key Concepts
-- **Disk Usage** (Default): The actual physical space consumed on disk (Blocks * 512 bytes). This is the "true" footprint and the metric `sized` prioritizes.
+- **Disk Usage** (Default): Filesystem-reported allocation (Blocks * 512 bytes), with hard links counted once per scan. Shared extents and snapshots can make this differ from the space recovered by deletion.
 - **Apparent Size**: The logical size of the entry (file length). Available via the `-a` or `--apparent` flag.
-- **Blocks**: The actual filesystem blocks allocated.
+- **Blocks**: Filesystem-reported 512-byte units of allocation. Directory metadata is included.
+
+Symlinks are scanned as leaves, including dangling links; their targets are not
+followed. Missing targets or unreadable entries are reported on stderr. Partial
+scans return a nonzero exit status and JSON includes `complete: false`; inspect
+that status before relying on a total.
+
+### Filesystem Boundary (`-x` / `--one-file-system`)
+
+Skip entries whose device differs from the scan root, useful when a directory
+contains mounted filesystems. Boundary entries contribute no size, and JSON
+marks them with `skipped_mount`.
+
+```bash
+sized -x /path/to/volume
+```
 
 ### Path Display
 - **Relative Path** (Default): `sized` shows paths relative to the current directory.
@@ -53,7 +68,7 @@ sized -d 1
 ```
 
 > [!NOTE]
-> Regardless of the display depth, `sized` always calculates the *total* size of all subdirectories accurately by traversing the entire tree.
+> Display depth limits output, not scanning. The tree is traversed unless filtered or excluded by a filesystem boundary; scan errors mark totals incomplete.
 
 ## Filtering and Sorting