fix: pad short rows in SetRows to the number of columns

SetRows stored rows with fewer cells than the header unchanged, so short
rows rendered fmt's %!s(MISSING) verb and an empty row combined with a
first-column formatter panicked on vals[0]. Pad short rows to the column
count, mirroring AddRow's documented behavior of rendering missing cells
as empty.

PR.md

@@ -0,0 +1,83 @@
+# fix: pad short rows in SetRows to the number of columns
+
+## Problem
+
+`SetRows` does not pad rows that have fewer cells than the table has columns.
+As a result, short rows render fmt's missing-argument verb, and an empty row
+combined with a first-column formatter panics:
+
+```go
+tbl := table.New("A", "B", "C").WithPadding(1)
+tbl.SetRows([][]string{{"x", "y"}})
+tbl.Print()
+// A B C
+// x y %!s(MISSING)
+```
+
+```go
+tbl := table.New("A", "B").
+	WithFirstColumnFormatter(func(f string, v ...interface{}) string { return fmt.Sprintf(f, v...) })
+tbl.SetRows([][]string{{}})
+tbl.Print() // panic: runtime error: index out of range [0] with length 0
+```
+
+`AddRow` already handles this: it allocates `make([]string, len(t.header))` per
+row, so short input renders the remaining columns empty. The documented
+behaviour ("Rows can have less cells than the total number of columns;
+subsequent cells will be rendered empty.") is not honored by `SetRows`.
+
+## Root cause
+
+`table.go`, `SetRows`:
+
+```go
+for _, row := range rows {
+	if len(row) > headerLength {
+		t.rows = append(t.rows, row[:headerLength])
+	} else {
+		t.rows = append(t.rows, row) // short rows stored as-is
+	}
+}
+```
+
+Short rows are stored unchanged. Later, `printRow` builds one value per cell of
+the row and passes them to `fmt.Fprintf` with a format string containing
+`len(t.header)` verbs, so missing cells become `%!s(MISSING)`. With a
+first-column formatter, `printRow` also indexes `vals[0]`, which panics for an
+empty row.
+
+## Fix
+
+Pad short rows to the number of columns, mirroring `AddRow`:
+
+```go
+switch {
+case len(row) > headerLength:
+	t.rows = append(t.rows, row[:headerLength])
+case len(row) < headerLength:
+	// pad short rows so every column is rendered, matching AddRow
+	padded := make([]string, headerLength)
+	copy(padded, row)
+	t.rows = append(t.rows, padded)
+default:
+	t.rows = append(t.rows, row)
+}
+```
+
+## Test
+
+Added `TestSetRowsShortRowsPadded` (asserts no `MISSING` verb and the exact
+padded output) and `TestSetRowsEmptyRowWithFirstColumnFormatter` (asserts no
+panic).
+
+- Gate (`GOTOOLCHAIN=go1.25.10`): `go test ./...` -> ok (PASS, exit 0)
+- `go vet ./...` -> exit 0
+- `go build ./...` -> exit 0
+
+Counterfactual: reverting `SetRows` makes `TestSetRowsShortRowsPadded` fail with
+`output contains missing-arg verb: "A B C \nx y %!s(MISSING)\n"`.
+
+Note: `gofmt -l table.go` reports the file, but this is pre-existing (the
+`Print()` block near line 258 is space-indented on the base commit) and
+unrelated to this change; the modified region is gofmt-clean, so unrelated lines
+were intentionally left untouched.