Split library_ui.rs into a module directory by area of behaviour

controller holds LibraryController and the window-sizing constants every
other module reads and writes through pub(super) fields, the same shape
collections_ui and develop already use. open is the launch-to-scan cycle and
the worker that checks the catalog file before either touches it. offline is
what of a collection is on this device and the prompt that offers to change
it. window fills the grid model from the catalog and drains the thumbnail
fetch, which is the piece the catalog-reads-are-proportional-to-what-changed
rule (docs/catalog.md §1) bears on most directly. sync is the background
passes that reach beyond the loaded window: the metadata sweep, the
whole-library thumbnail pass, and the exchange with the server.
ratings_keywords applies a judgement or a keyword to a selection and queues
the sidecar and XMP writes behind it. timeline is the capture-time sidebar
and the photographer's place together, kept in one file because a restored
place ends by moving the timeline marker and a scrub is a restore of one
instant, so most calls between the two would otherwise cross a module
boundary. grid wires the grid's own callbacks — the keyboard cursor,
cell-size zoom, the routes into and out of develop — and filter_bar wires
the rating, people, date and offline-scope filters, calling back into
whichever of the above owns the work a filter change triggers.

Extracted by item rather than by line range, so every doc comment and
TRACES/GESTURE annotation stayed attached to the code it describes; the
sorted set of TRACES/GESTURE lines in the new directory is identical to the
original file's. Tests moved with the code they exercise, including the
handful of fixtures — settle, model_of, with_catalog, zoom_cell, pinch_step
— that only one target module needed and so were not worth sharing through
a test_support module the way the other splits use one. Items that crossed
a new module boundary were widened from private to pub(super), narrower
than the whole-file access the original gave them; a few items already
pub(crate) for recovery_ui or presets stayed there rather than being
narrowed, since nothing needed them tightened further.

mod.rs re-exports the same surface library_ui:: callers used before, so
lib.rs and every other caller needed no change.

ui/dr-ui/src/library_ui/mod.rs

@@ -0,0 +1,56 @@
+//! TRACES: FR-CAT-4 | FR-NC-3 | NFR-P9
+//! Drives the library grid from scan and thumbnail workers.
+//!
+//! Owns the bridge between three background activities and one single-threaded
+//! event loop:
+//!
+//! - a **scan** worker walking the remote tree into the catalog
+//! - a **thumbnail** worker range-fetching previews for visible cells
+//! - the **grid model** Slint renders
+//!
+//! Nothing here blocks. Workers post through mpsc channels drained by Slint
+//! timers, which is the same shape [`crate::launch_ui`] uses for login.
+//!
+//! # Why thumbnails are fetched, not derived from the scan
+//!
+//! A scan yields paths and sizes, nothing visual. Each thumbnail costs its own
+//! range request, so they are fetched **only for cells the grid actually
+//! wants** — never for the whole library up front. On the reference library
+//! that is the difference between a few MB and ~370 GB (ARCH §6.7).
+//!
+//! Split into one module per area of behaviour (docs/dev/code-health.md CH-1):
+//! `controller` holds the running state every other module reads and writes
+//! through `pub(super)` fields; `open` is the launch-to-scan cycle and the
+//! worker that checks the catalog file before either touches it; `offline`
+//! is what of a collection is on this device and the prompt that offers to
+//! change it; `window` fills the grid model from the catalog and drains the
+//! thumbnail fetch; `sync` is the background passes that reach beyond the
+//! loaded window — the metadata sweep, the whole-library thumbnail pass, and
+//! the exchange with the server; `ratings_keywords` applies a judgement or a
+//! keyword to a selection and queues the sidecar and XMP writes behind it;
+//! `timeline` is the capture-time sidebar and the photographer's place,
+//! which move together closely enough to live in one file; `grid` wires the
+//! grid's own callbacks — the keyboard cursor, cell-size zoom, the routes
+//! into and out of develop; `filter_bar` wires the rating, people, date and
+//! offline-scope filters. `mod.rs` re-exports what the rest of the crate
+//! calls by `library_ui::` path, so nothing outside this module needs to
+//! change.
+
+mod controller;
+mod filter_bar;
+mod grid;
+mod offline;
+mod open;
+mod ratings_keywords;
+mod sync;
+mod timeline;
+mod window;
+
+pub use controller::LibraryController;
+pub use grid::wire;
+pub(crate) use open::{forget_catalog, show_catalog_now, start_rescan};
+pub use open::{open, reload};
+pub use ratings_keywords::paste_settings_to_selection;
+pub(crate) use ratings_keywords::start_sidecar_writes;
+pub use timeline::format_date;
+pub use window::refresh_thumbnail;