An anchor is a peg

`[:xform :anchor]` is deleted. `T(a)·M·T(-a)` is a transform conjugated by a
translation — "do M in a frame shifted by a" — and a parent already IS a shifted
frame, so an anchor was a peg written inline: one that could not be selected,
keyed, shared between nodes, or placed above a measured channel. Same expressive
content, strictly less reach. `node-test` asserts the two produce the same matrix.

Its two jobs split, and neither is a field on a node any more.

A pivot nobody chose is DERIVED PER DRAG and stored nowhere. `gesture/pivot` is
the middle of what the node draws — `pick/bounds-of`, the same call the stage
draws the selection box from, on the same frame — or the node's own origin when it
draws nothing. `gesture/about` solves for the position that holds that point
still, so a turn now writes `pos` as well as `rot`. Nothing is cached, so nothing
goes stale: the stored anchor was that same middle captured once at creation while
the box beside it was recomputed every render, so on anything edited since it was
made the cross and the box disagreed and the pivot was wrong. A symbol with more
than one node diverged on its first edit.

A pivot somebody chose is a PEG — `nest/peg`, an ordinary `:group` parent sitting
on the derived pivot with `:pinv` captured so nothing moves. It is the answer to
the three things a derived pivot cannot do: a pivot that persists (an arm about
its shoulder), a pivot that travels (a keyed `pos`), and a hand transform over a
measured one. The last was impossible before — `local`'s translation is
`pos − M·a`, so under a measured `M` writing an anchor moves the thing it was
meant to leave alone. `flow/freeze`'s `pivoted` pass knew this and skipped every
`node/measured?` node, which is exactly why the traced mouth pivoted about
(-234, -395) on a 320x200 stage: the top-left corner of the footage. That pass is
gone; there is no node a derived pivot can be missing from.

`demo/stage` is the one place the anchor did work a static `pos` cannot: `:scale`
is keyed, and the source's middle has to stay on its authored centre throughout.
It is now seven pegs, identical to the pixel.

Also: `events/ui`'s `fitted` rescales a dropped tracing right after placement, and
the anchor had been silently keeping the picture centred through that; it solves
for the middle explicitly now.

Schema 7. Nothing is converted, as in 6: every project is marked 7 and one still
carrying an anchor is refused by name, with what to do about it. Dropping an
anchor is pixel-exact wherever rotation and scale are the identity — everywhere a
freeze or a drop wrote one — but not on anything since turned by hand, and not at
all where `pos` is dense, so a conversion would be silent and wrong for exactly
the nodes somebody had placed themselves.

601 CLJS tests, 68 Django tests, and the onion and take browser suites pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HkinzDz1VtahZVsujAGRBD

clips/migrations/0016_an_anchor_is_a_peg.py

@@ -0,0 +1,42 @@
+"""Schema 7: an anchor is a peg.
+
+`[:xform :anchor]` is gone from the transform. `T(a)·M·T(-a)` is a transform
+conjugated by a translation — "do M in a frame shifted by a" — and a parent
+already is a shifted frame, so an anchor was a peg written inline: one that could
+not be selected, keyed, shared between nodes, or placed above a measured channel.
+A pivot nobody chose is now derived from what the node draws, per drag, and stored
+nowhere; a pivot to keep is a peg, an ordinary `:group` parent.
+
+Nothing is converted, as in schema 6. Every project is marked 7, and one that
+still carries an anchor is refused when it is opened, by name and with what to do
+about it — `node/problems` in the frontend.
+
+Not converted rather than not worth converting. Dropping an anchor is in fact
+pixel-exact wherever rotation and scale are the identity, since the anchor
+cancels out of the composition there — and that is everywhere a freeze, a drop or
+a new drawing wrote one. It is NOT exact on anything a hand has since turned or
+scaled, where the composed translation is `a + p - M·a`, and it cannot be made
+exact at all where `pos` is dense, because tier 2 is content-addressed and not
+rewritable here. A conversion would therefore be silent and right for most nodes
+and silent and wrong for exactly the ones somebody had hand-placed, which is the
+worse failure: a refusal names the document and says what to do.
+"""
+
+from django.db import migrations, models
+
+
+def forwards(apps, schema_editor):
+    apps.get_model("clips", "Project").objects.update(schema_version=7)
+
+
+class Migration(migrations.Migration):
+    dependencies = [("clips", "0015_tracing_images")]
+
+    operations = [
+        migrations.AlterField(
+            model_name="project",
+            name="schema_version",
+            field=models.PositiveIntegerField(default=7),
+        ),
+        migrations.RunPython(forwards, migrations.RunPython.noop),
+    ]

clips/models.py

@@ -250,7 +250,7 @@ class Project(models.Model):
         settings.AUTH_USER_MODEL, blank=True, related_name="shared_projects",
     )
     name = models.CharField(max_length=200, default="untitled")
-    schema_version = models.PositiveIntegerField(default=6)
+    schema_version = models.PositiveIntegerField(default=7)
     seq = models.PositiveBigIntegerField(default=0)
     palette = models.CharField(max_length=64, default="arthur/default")
     created = models.DateTimeField(auto_now_add=True)

clips/tests/test_api.py

@@ -446,7 +446,7 @@ class DocumentTests(TestCase):
         self.assertEqual(5, len(response.json()["written"]))
 
         loaded = self.client.get(f"/api/projects/{self.project.id}").json()
-        self.assertEqual(6, loaded["schema_version"])
+        self.assertEqual(7, loaded["schema_version"])
         self.assertEqual(1, len(loaded["clips"]))
         clip = loaded["clips"][0]
         self.assertEqual("c1", clip["cid"])