refactor: extract the Jobs view state, track selection by job ID

Phase 10 of the whole-project review (findings 5.1 and 5.2), folded into
the ROADMAP file-split item as that plan asks.

5.2 was a real defect. `selected` was an index into a snapshot of the jobs
slice, and every path that changed the slice patched it by hand. The one
path that could not — adopting a different jobs file, where the Service
replaces the whole list and the view only hears about it through the
refresh JobsLoaded triggers — left the details pane redrawing from an
index that belonged to the previous list, describing whichever job now sat
there (or clearing when the new list was shorter) while the list highlight
stayed put. The selection is now a job ID; rows are derived from it at
render time, and refresh ends by pointing the highlight at the selected
job, so the two can no longer disagree.

5.1: newJobsView was one 330-line constructor whose dozen closures shared
seven mutable locals. It is now a jobsView struct over a jobsViewState
that owns the snapshot, the folder filter, and the selection — the
invariant that used to be maintained by hand in five places lives in one
place — split across jobs_view.go (construction, refresh, layout),
jobs_view_state.go, jobs_view_list.go, and jobs_view_toolbar.go. The
folder-option rebuild that appeared verbatim in three handlers is one
method.

Behaviour that changed beyond the fix: switching the folder filter keeps
the current selection when the new filter still shows it, instead of
always jumping to the folder's first job.

Docs: ARCHITECTURE records the new file layout and the selection-by-ID
contract; ROADMAP drops jobs_view.go from the over-guideline table and
refreshes the other five numbers (finding 2.4); TESTS documents the new
state test file and the adoption regression test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/TESTS.md

@@ -471,11 +471,34 @@ widgets are assembled.
 | `TestJobListViewCompactConfigOpensCompact` | Verifies the persisted density is honoured at build time, not only after a tap. |
 | `TestJobsSidebarWidthIsItsContent` | Regression guard: nothing but the sidebar's own toolbar row imposes a width floor on it. |
 | `TestJobsSplitOpensAtTheSidebarWidth` | Verifies the derived split offset opens the divider at the sidebar's own width at the default window size — enough that the toolbar is never born clipped, and no more. |
-| `TestToolbarButtonRedrawsRowAndDetails` | Regression guard: with the duplicate refreshes removed from the handlers, `refreshView` alone must re-snapshot the jobs and repopulate the details pane. |
+| `TestToolbarButtonRedrawsRowAndDetails` | Regression guard: with the duplicate refreshes removed from the handlers, `jobsView.refresh` alone must re-snapshot the jobs and repopulate the details pane. |
+| `TestJobsViewSelectionSurvivesAJobsFileSwitch` | Regression guard: adopting a different jobs file replaces the whole list from the Service, and the refresh that follows must leave the details pane and the list highlight describing the same job — not redraw the pane from a row index that belonged to the previous list. |
 | `TestDetailCaptionWidthCoversEveryCaption` | Verifies every caption `metadataRows` returns fits the measured caption column, which is what makes the single row list self-enforcing. |
 
 ---
 
+### src/ui/jobs_view_state_test.go
+
+**Package:** `ui`
+
+Tests `jobsViewState`, the Jobs tab's model: the job/runtime snapshot, the
+folder filter, and the ID-based selection. No Fyne app is built — the state
+touches no widgets, so these run in milliseconds.
+
+| Test | Purpose |
+|------|---------|
+| `TestJobsViewStateSelectsTheFirstJob` | Verifies the opening state selects the first row, so the details pane is never blank when there is something to show. |
+| `TestJobsViewStateEmptyListSelectsNothing` | Verifies an empty job list leaves nothing selected and no row to highlight (`displayRow` = -1). |
+| `TestJobsViewStateSelectionFollowsTheJobNotTheRow` | Regression guard: a job removed above the selected one (through the Service, the way an external change reaches the view) must not slide the selection onto its neighbour — the selection is a job ID, and only its row moves. |
+| `TestJobsViewStateDropsSelectionWhenItsJobIsGone` | Verifies a selection whose job no longer exists falls back to the first visible row instead of describing whichever job inherited its position. |
+| `TestJobsViewStateApplyFilter` | Verifies the folder filter keeps a selection it still shows, moves it to the folder's first row when it does not, and that "No folder" matches the job without one. |
+| `TestJobsViewStateEmptyFilterSelectsNothing` | Verifies a filter matching no job is a filter choice, not an error state: nothing selected, nothing highlighted, and the selection returns when the filter is cleared. |
+| `TestJobsViewStateHiddenSelectionIsNotHighlighted` | Verifies a selected job the filter hides reports no display row rather than falling back to row 0, which would highlight an unrelated job. |
+| `TestJobsViewStateRuntimeIsNeverNil` | Verifies `runtime` returns an empty `JobRuntime` for a job the Service has none for, so callers need no nil check. |
+| `TestJobsViewStateJobAtRejectsRowsOutsideTheFilter` | Verifies row lookups are bounded by the filtered rows, which is what the list widget draws from. |
+
+---
+
 ### src/ui/history_view_test.go
 
 **Package:** `ui`