perf: cap the History list and fold column widths incrementally

History was appended to on every recorded run and never trimmed, and every
event re-sorted the whole slice and re-measured the Job, Detail and Log
columns across every row. The per-run cost therefore grew with the number of
rows, in exactly the mode the app is designed for: left in the tray for days.

The session History now keeps the newest maxHistoryRows (1000) records, the
way maxJobLogs caps a job's own activity list, and drops the oldest from the
front, zeroing the tail so a dropped record's full captured output is not
kept alive by the backing array. Column widths move into a historyLog value
that folds each new record into the current maxima instead of rescanning.
Widths only grow within a theme, so a column never narrows when a record ages
out; a theme change is the one case that still rescans, because every stored
width was measured at the old text size.

Measured with a throwaway benchmark over 5000 accumulated records: one
refresh went from 15.8 ms to 0.9 ms. At the new cap the full width rescan
alone costs 1.5 ms, so both halves of the fix carry weight.

Plan item 6 of docs/PROJECT_REVIEW_PLAN.md (finding 3.1).

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

src/ui/history_view_test.go

@@ -1,6 +1,7 @@
 package ui
 
 import (
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -144,7 +145,8 @@ func TestHistorySortToggleKeepsRowsInSync(t *testing.T) {
 		{Time: "2026-06-01 11:00:00", JobName: "B"},
 		{Time: "2026-06-01 12:00:00", JobName: "C"},
 	}
-	content, refresh := newHistoryView(&events)
+	log := newHistoryLog(events)
+	content, refresh := newHistoryView(log)
 	table, ok := content.Objects[0].(*widget.Table)
 	if !ok {
 		t.Fatal("history view does not wrap a table")
@@ -192,7 +194,7 @@ func TestHistorySortToggleKeepsRowsInSync(t *testing.T) {
 
 	// A new run arrives while the table is sorted newest-first: it must be
 	// counted and placed in the order currently on screen, not the build-time one.
-	events = append(events, event{Time: "2026-06-01 13:00:00", JobName: "D"})
+	log.add(event{Time: "2026-06-01 13:00:00", JobName: "D"})
 	refresh()
 	assertOrder("descending after refresh", "D", "C", "B", "A")
 
@@ -207,8 +209,7 @@ func TestHistoryCellTemplateIsPlainText(t *testing.T) {
 	testApp := test.NewApp()
 	defer testApp.Quit()
 
-	var events []event
-	content, _ := newHistoryView(&events)
+	content, _ := newHistoryView(newHistoryLog(nil))
 	table := content.Objects[0].(*widget.Table)
 	label, ok := table.CreateCell().(*widget.Label)
 	if !ok {
@@ -292,6 +293,96 @@ func TestHistoryColumnsFitTheirContent(t *testing.T) {
 	check("scaled theme")
 }
 
+// TestHistoryLogCapsRecords is the regression guard for the unbounded History
+// list: an app left in the tray records thousands of runs a day, each carrying
+// the run's whole captured output, so the list must drop the oldest instead of
+// growing forever.
+func TestHistoryLogCapsRecords(t *testing.T) {
+	testApp := test.NewApp()
+	defer testApp.Quit()
+
+	log := newHistoryLog(nil)
+	for i := 0; i < maxHistoryRows+25; i++ {
+		log.add(event{Time: "t", JobName: "Job " + strconv.Itoa(i)})
+	}
+	if len(log.records) != maxHistoryRows {
+		t.Fatalf("record count = %d, want capped at %d", len(log.records), maxHistoryRows)
+	}
+	if got, want := log.records[0].JobName, "Job 25"; got != want {
+		t.Errorf("oldest kept record = %q, want %q — the cap must drop from the front", got, want)
+	}
+	last := log.records[len(log.records)-1].JobName
+	if want := "Job " + strconv.Itoa(maxHistoryRows+24); last != want {
+		t.Errorf("newest record = %q, want %q", last, want)
+	}
+	// A list handed in above the cap is trimmed too, not only one grown into it.
+	oversized := make([]event, maxHistoryRows+10)
+	if got := len(newHistoryLog(oversized).records); got != maxHistoryRows {
+		t.Errorf("pre-filled log length = %d, want %d", got, maxHistoryRows)
+	}
+}
+
+// TestHistoryLogWidthsMatchAFullScan pins the incremental column widths: while
+// every measured record is still in the list, folding each one in as it
+// arrives must give exactly what rescanning every row would, or the cheaper
+// path would clip values the old one showed.
+func TestHistoryLogWidthsMatchAFullScan(t *testing.T) {
+	testApp := test.NewApp()
+	defer testApp.Quit()
+
+	log := newHistoryLog(nil)
+	for _, record := range []event{
+		{Time: "1", JobName: "A", Detail: "short", LogFile: `/logs/a.log`},
+		{Time: "2", JobName: "A moderately long job name", Detail: "a longer detail message", LogFile: `/logs/20260601-120000_SomeJobName.log`},
+		{Time: "3", JobName: "B", Detail: "s", LogFile: `/logs/b.log`},
+	} {
+		log.add(record)
+	}
+	if got, want := log.columnWidths(), historyColumnWidths(log.records); got != want {
+		t.Errorf("incremental widths = %v, want the full-scan widths %v", got, want)
+	}
+}
+
+// TestHistoryLogWidthsDoNotShrinkWhenRecordsAgeOut covers the other half of the
+// rule: widths only grow. Dropping the record that set a column's width must
+// not narrow the column, because the rows on screen were laid out against it.
+func TestHistoryLogWidthsDoNotShrinkWhenRecordsAgeOut(t *testing.T) {
+	testApp := test.NewApp()
+	defer testApp.Quit()
+
+	log := newHistoryLog(nil)
+	log.add(event{Time: "1", JobName: "A job name long enough to widen its column"})
+	widest := log.columnWidths()[2]
+	for i := 0; i < maxHistoryRows; i++ {
+		log.add(event{Time: "t", JobName: "x"})
+	}
+	if got := log.columnWidths()[2]; got != widest {
+		t.Errorf("Job column width = %v after the wide record aged out, want it held at %v", got, widest)
+	}
+}
+
+// TestHistoryLogRescansOnThemeChange guards the one case the incremental fold
+// cannot handle: every stored width was measured at the old text size, so a
+// theme change has to fall back to a full rescan.
+func TestHistoryLogRescansOnThemeChange(t *testing.T) {
+	testApp := test.NewApp()
+	defer testApp.Quit()
+
+	log := newHistoryLog([]event{
+		{Time: "1", JobName: "A moderately long job name", Detail: "a longer detail message"},
+	})
+	before := log.columnWidths()
+
+	testApp.Settings().SetTheme(test.NewTheme())
+	after := log.columnWidths()
+	if after == before {
+		t.Fatal("widths unchanged after a theme change; the fixture theme must alter text metrics")
+	}
+	if want := historyColumnWidths(log.records); after != want {
+		t.Errorf("widths after theme change = %v, want the rescanned %v", after, want)
+	}
+}
+
 func TestNewEventUsesConsistentTimestampShape(t *testing.T) {
 	ev := newEvent(1, "Job", "OK", "detail")
 	if _, err := time.Parse("2006-01-02 15:04:05", ev.Time); err != nil {