handoff: F6 -- sweep gated at t=70/100, and reinstate rate 0.5

Gives the port the declared parent ramp and corrects my withdrawal of the
0.514 rate: the frames-based expression was the error, not the ratio.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Jc4pciRArGHfxGGhEbwp5t

docs/port/HANDOFF.md

@@ -6056,3 +6056,31 @@ reads **1.0 units/frame** (`ptcopyright`, 22-unit ramp) and **0.571** (plate,
 declared 12-unit ramp) — 1.75× apart. Either a declared ramp is misread or the
 two are **not on one clock**, i.e. `clock: "shared"`. That is F4. Ratios above
 need no clock and stand; the title-unit conversions wait on it.
+
+## F6 — the sweep is gated by its parent's declared alpha (`fa99b5f`)
+
+`docs/re/f6-unit10-parent-alpha-gates-the-sweep.md`.
+
+**`GP_TITLE` build 4/7 declare `ptloop01.rat`/`ptloop02.rat`:**
+`t=0 α0 · t=70 α0 · t=100 α255 · t=238 α255 · t=250 α0`
+
+So: **sweep off until `t=70`, full at `t=100`.** `238…250` is an **exit** ramp,
+not an entry — it never plays, because the title clock freezes inside `[160,236]`.
+⚠️ Builds **5 and 6** declare the same records **flat α255**; name the build.
+
+**The parent's alpha multiplies into the leaf's own.** Measured: with the leaf's
+declared curve divided out, the implied parent pins at **255.0 (±1.5)** across
+hundreds of frames while the drawn alpha swings 242→132→145. The standing 🟡 on
+this is closed — multiply, don't override.
+
+🔴 **Reinstate `rate = 0.5`.** I told you to withdraw unit 8's 0.514 last
+iteration; that was wrong and it is on me. What failed was quoting rates in
+*captured frames*; the ratio was sound. Two runs that differ 2× in frames both
+give **leaf/title = 0.4795 and 0.4667** — 2.7% apart — against the parent's
+declared 30-unit ramp. `rate: null` (screen rate) makes the sweep cross twice
+too fast.
+
+🟡 Unaffected by this but still open: the element I called "the plate" is
+identified by position only, and lands at `t≈137` where the declared plate is
+`t≈236`. Unit 9's onset *fraction* is measured against it and inherits that
+doubt. The `t=70`/`t=100` numbers above are declared and do not.