feat(layout): add fit_text_proportional — proportional sizing vs. always-maximize

fit_text always picks the largest ladder rung that fits its box. That's
right when an element owns dedicated space, but wrong when several
independently-fitted elements need to stay visually harmonious as the
panel grows: a score's box might have generous room while a neighboring
logo scales by a fixed geometry factor via px() — fit_text lets the score
balloon out of proportion (even overlapping the logo) even though its
individual pick is technically correct.

fit_text_proportional(text, box, base_size_px, ladder) instead targets
base_size_px * self.scale (the same scale factor px() already uses),
picking the nearest ladder rung at or below that target, still capped to
what fits the box, floored at the smallest rung when the target is below
every rung. Refactored the shared largest-that-fits/ellipsize walk into
_walk_ladder() so fit_text and fit_text_proportional don't duplicate it.

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

test/test_adaptive_layout.py

@@ -233,6 +233,47 @@ class TestFontFitting:
         ctx.clear_cache()
         assert ctx.fit_text("CACHED", ctx.bounds) is not first
 
+    def test_fit_text_proportional_tracks_design_scale(self, font_manager):
+        # design size 128x32, base_size_px=10 (a typical classic score size):
+        # at 2x scale the target is 20px -> nearest LADDER_ARCADE rung <= 20
+        # is 16px, not the largest that merely fits the box (32).
+        ctx = LayoutContext(256, 64, font_manager)  # scale = min(2,2) = 2
+        fit = ctx.fit_text_proportional("17-21", ctx.bounds, base_size_px=10,
+                                        ladder=LADDER_ARCADE)
+        assert fit.size_px == 16
+
+    def test_fit_text_proportional_does_not_exceed_max_fit(self, ctx):
+        # at scale=1 (128x32, the design size itself) the target equals
+        # base_size_px, so proportional should never pick something LARGER
+        # than plain fit_text would for the same box.
+        prop = ctx.fit_text_proportional("17-21", ctx.bounds, base_size_px=10,
+                                         ladder=LADDER_ARCADE)
+        maxed = ctx.fit_text("17-21", ctx.bounds, ladder=LADDER_ARCADE)
+        assert prop.size_px <= maxed.size_px
+
+    def test_fit_text_proportional_floors_at_smallest_rung(self, font_manager):
+        # scale so small the target is below every rung -> use the smallest
+        # rung as a floor rather than refusing to render anything.
+        ctx = LayoutContext(32, 8, font_manager)  # scale = min(32/128, 8/32) = 0.25
+        fit = ctx.fit_text_proportional("HI", ctx.bounds, base_size_px=10,
+                                        ladder=LADDER_ARCADE)
+        assert fit.size_px == min(s.size_px for s in LADDER_ARCADE)
+
+    def test_fit_text_proportional_falls_through_when_target_rung_overflows(self, font_manager):
+        # a long string at the target rung might not fit a narrow box even
+        # though the target size is "correct" -- must fall through to a
+        # smaller rung exactly like fit_text does, not just refuse to fit.
+        ctx = LayoutContext(256, 64, font_manager)
+        narrow_box = Region(0, 0, 40, 64)
+        fit = ctx.fit_text_proportional("A REALLY LONG STRING HERE", narrow_box,
+                                        base_size_px=10, ladder=LADDER_ARCADE)
+        assert fit.fits or fit.text.endswith("…")
+
+    def test_fit_text_proportional_cached(self, ctx):
+        first = ctx.fit_text_proportional("X", ctx.bounds, base_size_px=10)
+        second = ctx.fit_text_proportional("X", ctx.bounds, base_size_px=10)
+        assert first is second
+
     def test_fit_lines_stacks_within_height(self, ctx):
         box = ctx.bounds
         lines = ["LINE ONE", "LINE TWO", "LINE THREE"]