refactor(card): audit title-fit logic — O(1) form + lock-in unit test (#12)

The title fitter was correct but the loop-down-from-maxFont form hid
its intent. Switch to the direct expression:

    ideal = floor(budget / (chars * 0.6))
    clamp to [11, 15]

Same answer for every title; easier to verify at a glance. Hoist the
four constants (leftInset=20, rightSafety=4, minFont=11, maxFont=15,
charRatio=0.6) to package scope so the new unit test can reference
them without re-declaring.

Utilization audit for realistic dracula titles (width=340, budget=316):

  Stats (5)                                        → 15 px (14 %)
  Top Starred Repos (17)                           → 15 px (48 %)
  Most Commit Language (all time) (31)             → 15 px (91 %)
  Commits by Hour (last year, UTC+7.00) (37)       → 14 px (98 %)
  Commits by Weekday (last year, UTC+7.00) (40)    → 13 px (99 %)
  Commits by Weekday (last year, UTC+12.75) (41)   → 12 px (93 %)
  200-char pathological                            → 11 px (floor)

TestFitTitleFontSize pins this table so a future charRatio tweak can't
silently regress any real title. TestCardsFitFrame (the end-to-end
check) plus this unit test now cover both the geometry and the picked-
font-size paths.

internal/card/card_test.go

@@ -158,6 +158,35 @@ func TestFormatInt(t *testing.T) {
 	}
 }
 
+// TestFitTitleFontSize locks in the title-sizing behavior for the set of
+// realistic titles the renderers actually emit. Any regression that shrinks
+// a short title below 15 px, or lets a 40-char title stay at 15 px when it
+// would overflow, will fail this test.
+func TestFitTitleFontSize(t *testing.T) {
+	const width = 340
+	cases := []struct {
+		title string
+		want  int
+	}{
+		{"", 15},
+		{"Stats", 15},
+		{"Streak", 15},
+		{"Top Starred Repos", 15},
+		{"Most Commit Language (all time)", 15},             // 31 chars
+		{"Contributions by Year", 15},
+		{"Commits by Hour (last year, UTC+7.00)", 14},       // 37 chars
+		{"Commits by Weekday (last year, UTC+7.00)", 13},    // 40 chars
+		{"Commits by Weekday (last year, UTC+12.75)", 12},   // 41 chars
+		{strings.Repeat("x", 200), 11},                       // pathological
+	}
+	for _, c := range cases {
+		got := fitTitleFontSize(c.title, width)
+		if got != c.want {
+			t.Errorf("fitTitleFontSize(%q)=%d want %d", c.title, got, c.want)
+		}
+	}
+}
+
 // TestNiceTicksCoversMax guards the key invariant: the last tick returned
 // must be ≥ the requested max, or bar-chart cards render bars taller than
 // the chart area and poke into the title. Regression case: max=625 step=100

internal/card/svg.go

@@ -2,6 +2,7 @@ package card
 
 import (
 	"fmt"
+	"math"
 	"strings"
 	"unicode/utf8"
 )
@@ -81,29 +82,46 @@ func header(width, height int, bg, stroke string, strokeOpacity float64, titleCo
 		fontSize, titleColor, escapeXML(title))
 }
 
-// fitTitleFontSize picks the largest title font (between 11 and 15 px) at
-// which the title still fits in width − leftInset − rightSafety. Uses the
-// same 0.6 × font-size char-width estimate as the fit-the-frame test.
+// fitTitleFontSize returns the largest integer font size in
+// [titleMinFont, titleMaxFont] at which the title still fits in
+// width − titleLeftInset − titleRightSafety, using the same 0.6 × fontSize
+// char-width estimate as the fit-the-frame test. Computed directly from
+// the budget: ideal = budget / (chars × 0.6), floored to an int, clamped
+// to the allowed range.
+//
+// Utilization for realistic dracula titles (width=340, budget=316):
+//
+//	"Stats" (5)                                        → 15 px (14 %)
+//	"Top Starred Repos" (17)                           → 15 px (48 %)
+//	"Most Commit Language (all time)" (32)             → 15 px (91 %)
+//	"Commits by Hour (last year, UTC+7.00)" (37)       → 14 px (98 %)
+//	"Commits by Weekday (last year, UTC+7.00)" (40)    → 13 px (99 %)
+//	"Commits by Weekday (last year, UTC+12.75)" (41)   → 12 px (93 %)
 func fitTitleFontSize(title string, width int) int {
-	const (
-		leftInset    = 20
-		rightSafety  = 4
-		minFont      = 11
-		maxFont      = 15
-		avgCharRatio = 0.6
-	)
-	budget := float64(width - leftInset - rightSafety)
 	chars := utf8.RuneCountInString(title)
 	if chars == 0 {
-		return maxFont
+		return titleMaxFont
 	}
-	for fs := maxFont; fs >= minFont; fs-- {
-		if float64(chars)*float64(fs)*avgCharRatio <= budget {
-			return fs
-		}
+	budget := float64(width - titleLeftInset - titleRightSafety)
+	ideal := int(math.Floor(budget / (float64(chars) * titleCharRatio)))
+	if ideal > titleMaxFont {
+		return titleMaxFont
 	}
-	return minFont
+	if ideal < titleMinFont {
+		return titleMinFont
+	}
+	return ideal
 }
 
+// Title-sizing constants are exported to package-private so the unit test
+// can reference them without duplicating magic numbers.
+const (
+	titleLeftInset    = 20
+	titleRightSafety  = 4
+	titleMinFont      = 11
+	titleMaxFont      = 15
+	titleCharRatio    = 0.6
+)
+
 const footer = `
 </svg>`