docs(admin): scanning page — the shallow vs. force taxonomy from source

First page of the admin tree, written to the new admin tier (what the
click actually does). Documents every scan surface with its verified
semantics: Force Rescan All Libraries (dashboard, force, all libraries,
one job each with skip-on-active), the per-library Scan button (ALSO a
force scan — hx-vals force:true since the button's birth commit 706be09d,
which corrects the owner's stated 'other scan buttons are shallow'
belief; flagged to the owner), Scan this folder (shallow, single-folder
scope, archive pass skipped for partial scans, podcast roots refused),
Auto-Scan + Scan Interval (the watcher + poll + startup triggers, all
shallow, default 60s range 1-3600), Processing Issues retry (per-item
sequential, deliberately not a library scan), and book-level Rescan /
Reset to Scanned. Plus the standing rules: one scan per library
(coalescing), background jobs with the dashboard progress panel, and
the two-consecutive-scans archive rule.

Glossary gains Scan and Scan this folder (same-commit rule for controls
cited in prose). The real-tree nav test's Administration assertion now
pins shape (Admin Guide, Libraries prefix + usabilityOrder subsequence)
instead of an exact slice, so pages landing one at a time don't churn it.

docs/developer/ui-controls.md

@@ -218,6 +218,14 @@ does: Force-rescans every library — re-reads each file, re-extracts metadata w
 also: Scan All Libraries
 note: renamed 2026-11-02 — the old label under-promised a heavy operation
 
+label: Scan
+where: Admin → Libraries → a library's card row
+does: Force-rescans that one library (same semantics as Force Rescan All Libraries, scoped to a single library).
+
+label: Scan this folder
+where: Admin → Libraries → a library's panel → a folder row
+does: Shallow-scans only that folder's subtree — unchanged files are skipped, and items in the library's other folders are never marked missing by a partial scan.
+
 label: Resume rewind
 where: audio player → player settings; profile settings
 does: Rewinds playback a configured few seconds when you resume.