feat(design-systems): declare what to write instead, rather than what not to

Milestone #254 step 6, first half (#2295) — and this reframes the task rather
than answering it. The operator's call:

  "in this case we should declare what should be used in place of pure white,
   it's not a prohibition it's what should be used in its place."

None of the three options on the table (a constraints record / the panel reads
both sources / negative token rows) was right, because all three kept the
prohibition as a KIND OF THING. It isn't one. "Pure white is never text" is the
shadow cast by a positive fact — text is Parchment — and a design system that
stores what things ARE has no row for a ban because it never needed one.

So `design_tokens` gains `supersedes`: the literal values this token should be
written instead of. `--color-text-on-action` supersedes `#fff` / `#ffffff`. Same
fact as the rule, stated forwards, and now actionable — a finding can say what
to write rather than only objecting.

It has to be DECLARED, not derived, and that is the crux: `#fff` and Parchment
`#E8E4D8` are different colours, so no value-matching check could ever have
connected them. That mismatch is precisely why the prohibition looked
unrepresentable until it was turned around.

`supersedes` cascades on EMPTINESS rather than on None. A child overriding a
colour says nothing about which literals it replaces, and blanking the family's
declaration there would silently disarm the check for every app that customises
the token — while a child that states its own list replaces it wholesale.

Two things this deliberately does NOT do:

  - It does not feed the drift panel. Superseded literals live in component CSS,
    which `designDrift.ts` cannot see and already documents as a blind spot.
    This is input for the source lint (#2277). Declaring it with nothing
    consuming it yet is honest; wiring it to a panel that cannot check it would
    not be.
  - It does not remove the panel's `prohibited_color` arm yet — that happens
    when the panel is repointed at a resolved system, which needs #2288 first.

The declaration also exposes a missing token: most of the 67 hardcoded
`color: #fff` (#2275) are text on a coloured action button, and the system has
no token for that role at all. Every view hardcodes it. Declaring the token that
was never there is the first real output of the operator's framing.

src/scribe/models/design_system.py

@@ -109,6 +109,24 @@ class DesignToken(Base, TimestampMixin, SoftDeleteMixin):
     # vocabulary, and a whitelist would bake one install's kit into the schema.
     group_name: Mapped[str | None] = mapped_column(Text, nullable=True)
     purpose: Mapped[str | None] = mapped_column(Text, nullable=True)
+    # Literal values this token should be used INSTEAD OF, e.g. ["#fff",
+    # "#ffffff"] on a text-on-action token.
+    #
+    # This is how a design system records the thing a prohibition was trying to
+    # say. "Pure white is never text" is the shadow of a positive fact — text is
+    # Parchment — and a system that stores what things ARE has no row for a ban.
+    # Recording the replacement keeps the check and makes it actionable: a
+    # finding can name what to write instead of merely objecting.
+    #
+    # It has to be DECLARED rather than inferred, because the superseded literal
+    # and the token's own value are usually different colours (#fff is not
+    # #E8E4D8). No value-matching rule could ever connect them.
+    #
+    # Consumed by the source lint (#2277), not by the drift panel: these
+    # literals live in component CSS, which the panel cannot see and says so.
+    supersedes: Mapped[list] = mapped_column(
+        JSONB, nullable=False, default=list, server_default=text("'[]'::jsonb")
+    )
     order_index: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
 
     def to_dict(self) -> dict:
@@ -119,6 +137,7 @@ class DesignToken(Base, TimestampMixin, SoftDeleteMixin):
             "value_by_mode": self.value_by_mode or {},
             "group_name": self.group_name,
             "purpose": self.purpose,
+            "supersedes": self.supersedes or [],
             "order_index": self.order_index,
             "created_at": self.created_at.isoformat() if self.created_at else None,
             "updated_at": self.updated_at.isoformat() if self.updated_at else None,