formats: teach the renderer to rotate (Option A) -- and report that it does not close the title

The human chose Option A: teach sylpheed-formats own renderer to draw
rotation_deg so it and the port stay comparable and verify-screen keeps meaning
someone is wrong.

Three pieces, because rotation alone does nothing on the title. blit gains a
rotated path that draws by inverse mapping over the rotated bounding box, turning
about the pivot, whose absolute position is invariant under scale; zero rotation
keeps the original forward-mapped path byte for byte so non-rotating screens
cannot regress. compose draws a nested .rat leaf when the leaf carries geometry
the parent does not, which is the sweeps case, but not as a blanket rule since a
button s leaf duplicates its parent. And --at poses leaves at a keyframe time,
because the sweeps hold off-screen at x=1521 so a resting composite omits them.

A trap found the hard way: posing EVERYTHING at one global time is wrong, because
a top-level group s final keyframes are its exit ramp and rest() deliberately
stops before them. Posing the title at t=358 walked every parent into its exit
and drove the disagreement from 10.92 to 61.74. So at poses leaves only.

Controls: 0 and 360 degrees byte-identical to the unrotated path, 90 degrees
swaps a 10x4 to 4x10, area conserved within 15 percent, centroid stays on the
pivot. 116 lib tests pass, main_menu unchanged at 9.26.

And the verification did not show what it was meant to, which is reported rather
than buried: scanning the pose time against the title capture gives 10.73 to
11.17 against a 10.92 baseline -- flat, no minimum, best 1.7 percent. The
whole-frame mean is dominated by the tone curve, and the renderer still does not
draw ptlogo1/ptlogo2 at all, which is a far larger spatial gap than two
translucent sweeps. So rotation is correct in isolation and no screen regressed,
but whether it closes the port s 1.81 percent is not established here.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QsEPXWVaEpyfudtR6re1Pd

docs/re/structures/ui-rotation-implemented.md

@@ -0,0 +1,90 @@
+# ✅ Option A implemented — the reference renderer rotates. ⚠️ It does not close the title.
+
+**Decision:** the human chose **Option A** (2026-08-29) — teach
+`sylpheed-formats`' own renderer to draw `rotation_deg`, so it and the port stay
+comparable and `verify-screen` keeps meaning *"someone is wrong"*.
+
+**Status:** ✅ implemented and controlled. 🔴 **and it does not measurably improve
+the title against the capture we hold** — reported here rather than quietly, because
+the improvement was the reason for doing it.
+
+## What changed
+
+Three pieces, because rotation alone does nothing on the title:
+
+1. **`blit` gained a rotated path.** `rotation_deg != 0` draws by **inverse
+   mapping** over the rotated bounding box; forward-mapping a rotation leaves
+   gaps. Rotation turns about the element's **pivot**, whose absolute position
+   `(kf.x + pivot_x, kf.y + pivot_y)` is invariant under scale.
+   ✅ `rotation_deg == 0` keeps the original forward-mapped path **byte for
+   byte**, so screens that do not rotate cannot regress.
+2. **`compose` draws a nested `.rat` leaf when the leaf carries geometry the
+   parent does not** — the title's sweeps are exactly that case (parent fixed at
+   (441,270) scale 100 %, leaf holding 600 %/800 % and ±30°/−45°).
+   ⚠️ Not a blanket rule: a button's leaf *duplicates* its parent and the parent
+   wins ([`ui-leaf-vs-parent-alpha.md`](ui-leaf-vs-parent-alpha.md)), so the leaf
+   is used only when its pose genuinely differs.
+   ⚠️ A leaf element resolves **no sprite of its own** — names resolve against
+   the bundle a build was parsed from, and a leaf is parsed from its own slice.
+   Its *name* is the sprite name, looked up in the parent bundle's table.
+3. **`--at <units>` / `ComposeOptions::at`**, because the sweeps hold off-screen
+   at `x = 1521` and a resting composite therefore *omits* them.
+
+## 🔴 A trap this found, and it cost a wrong number first
+
+Posing **everything** at one global time is wrong. A top-level group's final
+keyframes are its **exit ramp** — the fade-out played when the screen leaves —
+and `rest()` deliberately stops at the last *hold* keyframe before it. Posing the
+title at t=358 walked every parent into its exit and drove the disagreement from
+**10.92 to 61.74**.
+
+✅ `at` therefore poses **leaves only**; top-level elements keep `rest()`. That
+follows the decoded rule directly: the leaf runs on its own timeline and the
+parent's does not gate it.
+
+## The controls
+
+| | |
+|---|---|
+| 0° and **360°** vs the unrotated path | **byte-identical** |
+| 90° on a 10×4 sprite | extents swap to **4×10** |
+| covered area under rotation | conserved to **< 15 %** |
+| centroid under rotation | stays on the pivot (< 1 px) |
+
+`rotation_control_known_angles` pins all four. **116 lib tests pass.**
+
+## 🔴 The verification, which did not show what it was meant to
+
+Rendering the title against
+[`live-title-build4-no-plate.png`](../captures/title-builds/live-title-build4-no-plate.png)
+and scanning the pose time:
+
+| | mean abs difference |
+|---|---|
+| before (rest, no leaves, no rotation) | **10.92** |
+| after, scanned t = 0 … 600 | **10.73 – 11.17** |
+| best (t = 420) | 10.73 — **1.7 %** better |
+
+**Flat. No minimum.** Drawing the sweeps correctly does not measurably improve
+this comparison, and two things explain why without rescuing it:
+
+* the whole-frame mean is dominated by the **tone curve**, which
+  [`title-residual-tone-vs-geometry.md`](title-residual-tone-vs-geometry.md)
+  measures as the larger part of the *level* difference even where geometry is
+  right;
+* our renderer still **does not draw `ptlogo1` / `ptlogo2` at all** (four
+  elements, reported as "not drawn"), and that is a far larger spatial gap than
+  two translucent sweeps.
+
+⚠️ **So the honest claim is narrow:** rotation is implemented and correct in
+isolation, and no screen regressed. Whether it closes the port's **1.81 % of
+pixels differing** is **not established here** — that harness poses deliberately
+and counts differing pixels rather than mean level, and it is the place to judge
+it. ❔ The sweeps may simply be a small term.
+
+## No regression
+
+| screen | before | after |
+|---|---|---|
+| `main_menu` | 9.26 | **9.26** |
+| `extras` | — | 9.75 |