Name the app's Iris line app-pin, and let main mean the PR's base

The `iris` submodule's `main` was the fork's own line -- the app's 45
commits -- and not an ancestor of upstream's main, so it sat four merged
pull requests behind PR #19's actual base. Every obvious command was
therefore wrong in a way that looked plausible: `git diff main...` showed
#10, #12, #16 and #17 as the review branch's work, which is how the parley
text migration's undo path kept being reported as #19's.

The app's line is now `app-pin`, pushed to origin, and `main` tracks
`upstream/main`. `.gitmodules` follows `app-pin`, so `submodule update
--remote` cannot drag the pin onto upstream. The recorded commit is
unchanged at 32f6ad8; only the name it is reachable by has changed.

`origin/main` on the fork is left alone: moving it is a non-fast-forward on
a shared branch.

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

docs/LAYOUT_LOG.md

@@ -12,7 +12,7 @@ Over what the five earlier rounds did not name -- the WGSL prelude and how
 it is assembled, the position widgets, `orientation/`, and the sensor walk.
 **Scoped against `upstream/main` at `ca2b4b2`, which is PR #19's real base**;
 the first half of this sweep used the local `main` and had to be redone, for
-which see "The base is `upstream/main`" in `docs/HANDOFF.md`. Two findings.
+which see "The branch layout" in `docs/HANDOFF.md`. Two findings.
 The cold dump is byte-identical to `1096c31` and all three seed scans pass
 (400 at depth 5 in 69.07s, 1,000 at depth 6 in 169.29s, 2,000 at depth 4 in
 300.75s).
@@ -175,7 +175,7 @@ throughout, `LayoutLen::{is_px, is_only_leftover, declared, fills}` and
   event including the arrow keys. **`apply_event` is not in this PR's diff at
   all**: it was last touched by #10 and #16, both already on `upstream/main`.
   It keeps resurfacing in sweeps because they diffed against the local `main`
-  -- see "The base is `upstream/main`" in `docs/HANDOFF.md`. Bryan wants the
+  -- see "The branch layout" in `docs/HANDOFF.md`. Bryan wants the
   unbounded push and the clone dealt with as a change of their own
   (2026-09-20).
 - `ActivationState::update` writes four arms where the `Start`/`On` and