fix(layout): bound the fit cache; never alias the source image in fits

Two latent issues found in a self-review pass:

- LayoutContext._fit_cache was an unbounded dict (the image cache got an
  LRU cap, the text-fit cache didn't). Cache keys embed the fitted TEXT,
  so a plugin fitting changing strings — a live game clock, a ticker —
  on a 24/7 service grows it forever. Now LRU-bounded at 512 entries via
  the same pattern as the image cache.

- fit_image returned the caller's ORIGINAL image object when the source
  was already RGBA at target size (contain/fill_height, no ink crop).
  ImageFitResult is documented as an independent copy, and LayoutContext
  caches results — an aliased image lets later mutations of the source
  corrupt cached fits (or vice versa). Copy in that branch.

Both covered by new regression tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FqzC1nzTWL4kaqgMaQZFam

test/test_adaptive_layout.py

@@ -436,3 +436,19 @@ class TestBasePluginIntegration:
                          MockCacheManager(), pm)
         assert plugin.layout.design_size == (64, 32)
         assert plugin.layout.scale == 2.0
+
+
+class TestFitCacheBound:
+    def test_fit_cache_is_lru_bounded(self, ctx):
+        """A plugin fitting changing text (live game clock, ticker) on a
+        24/7 service must not grow the fit cache without bound."""
+        for i in range(ctx._FIT_CACHE_MAX + 100):
+            ctx.fit_text(f"tick {i}", Region(0, 0, 100, 20))
+        assert len(ctx._fit_cache) <= ctx._FIT_CACHE_MAX
+
+    def test_lru_keeps_recent_entries_hot(self, ctx):
+        hot = ctx.fit_text("stay hot", Region(0, 0, 100, 20))
+        for i in range(ctx._FIT_CACHE_MAX - 1):
+            ctx.fit_text(f"cold {i}", Region(0, 0, 100, 20))
+            ctx.fit_text("stay hot", Region(0, 0, 100, 20))  # keep touching it
+        assert ctx.fit_text("stay hot", Region(0, 0, 100, 20)) is hot