port: F6 unit a -- what starts the title sweep, and the gate that does not exist

No behaviour change. The start mechanism is one expression, screen_view.gd:684:
  var t := leaf_time_units if leaf_time_units >= 0.0 else time_units
leaf_time_units is set only by the --leaf-time diagnostic and is -1.0 on every
real boot, so the sweep rides the title's own clock from 0 with no offset.

Refuted the obvious reading, which was mine before checking: ptloop01/02 declare
0:0 70:0 100:255, so the parents LOOK like the gate. They are not -- the leaf is
what reaches the screen and the parent's alpha is decoded as not multiplied in.
pteff03's leaf declares 0:255 and is travelling from t=0; the plate arrives at
t=214-236, so the port starts the sweep ~150 units (~2.5 s) early, which is the
size and direction the human reported.

A start time is an OFFSET and leaf_time_units is an ABSOLUTE override; the
one-line edit is a subtraction at 684 fed by one authored value.

docs/port/f6-what-starts-the-sweep.md

@@ -0,0 +1,72 @@
+# F6 unit a — what the port currently uses to start the title sweep
+
+**Status:** ✅ answered. **No behaviour changed** — this unit exists so that when
+the Decoder says *when* the glow should start, the edit is one line.
+Port at `937f055`, 2026-09-02.
+
+## The answer, in one line
+
+`port/scripts/screen_view.gd:684`
+
+```gdscript
+var t := leaf_time_units if leaf_time_units >= 0.0 else time_units
+```
+
+**That is the whole start mechanism, and it is not a start mechanism.**
+`leaf_time_units` is set in exactly one place — `boot.gd:359`, the `--leaf-time`
+diagnostic flag — and is `-1.0` on every real boot. So the travelling glow runs
+on `view.time_units`, the title screen's own clock, which `_advance` sets to
+`0.0` when the title is raised. **Zero offset, no gate.**
+
+## 🔴 And the obvious gate does not exist
+
+The natural reading — mine, before checking — is that the parents gate it:
+`ptloop01`/`ptloop02` declare `0:0 70:0 100:255 238:255 250:0`, invisible until
+t=70. **That is not what happens**, because what reaches the screen is the LEAF,
+and `screen_view.gd` records as decoded that *"the leaf runs on its OWN timeline
+and the parent's alpha is NOT multiplied in"*. The parent ramp gates nothing.
+
+The leaves' own declarations:
+
+| leaf | alpha | x position |
+|---|---|---|
+| `pteff03` | **`0:255`** 150:128 540:255 600:255 | −639 → −39 (t=150) → 1521 (t=540) |
+| `pteff03a` | 0:0 150:128 630:255 720:255 | 1721 → 1111 (t=150) → −839 (t=630) |
+
+**`pteff03` is at full alpha from title t=0** and is travelling from t=0. It
+clears the left edge (sprite is 399 wide) at around t≈60 and is well inside the
+frame by t=150.
+
+The plate arrives at **t=214–236**. So the port starts the sweep roughly
+**150+ units ≈ 2.5 s early** — which is the size and the direction of what the
+human reported.
+
+## Corroborated on a film, not only read
+
+Filmed a real boot at 0.05 s and measured frame-to-frame change in the title art
+band `1280x420+0+90`, which **excludes the plate's own rectangle** (y 550–600) so
+the plate cannot be what registers:
+
+```
+view_units   22    54    69    86   118   134   214   341   406
+delta      31.8  39.9  30.8   9.4  12.6   0.2   0.2   0.1   0.4
+```
+
+Motion is heavy through the build-in and the band is quiet by t≈134 — consistent
+with `pteff03` having already crossed the measured band and with the coarse
+resize washing a thin glow out. **The film neither adds to nor contradicts the
+declaration; the declaration is the evidence here.**
+
+## What changes when the answer lands
+
+A start time is an **offset**, and `leaf_time_units` is an absolute override —
+they are not the same field. The one-line edit at 684 becomes a subtraction, fed
+by one authored value. Nothing else moves.
+
+## What this does NOT do
+
+* **It does not choose a start time.** That is the Decoder's, and this unit was
+  scoped to exclude it deliberately.
+* It does not touch the glow. A boot looks exactly as it did.
+* It says nothing about whether the *speed* or the *path* is right — only when it
+  begins.