feat(design-systems): resolve the chain, and keep the argument not the verdict

Milestone #254 step 2 (#2287). `resolve_tokens` flattens a system's inheritance
chain into its effective token set — walk to the root, deepest wins by token
name. Pure and duck-typed, so a test states a whole hierarchy in literals and
the service hands the same function ORM rows.

**Provenance is stored as the contest, not the winner.** A ResolvedToken carries
every system that offered a value, per mode, deepest first — `[0]` won and
`[1:]` are what it shadowed. "Which system supplied this?" and "what did it
override?" are then two reads of one list and cannot disagree, where a winner
plus a separate provenance field would be two things to keep in step.

**Merging is per (name, MODE), and that is the storage decision paying off.** A
system that deepens one accent for light backgrounds while leaving dark alone
owns `base` and still inherits `dark`. A token-level "overridden here" flag
would have to lie about one of them, and the two-column shape could not have
represented it at all.

Metadata cascades separately by the same deepest-wins rule, with one exception:
`order_index` treats 0 as UNSTATED rather than "first", because 0 is the column
default. Reading it as a real value would let a colour-only override drag its
token to the top of its group — a visible reshuffle in return for a change that
touched nothing structural.

One fix to step 1 while wiring this up: `_parent_map` is now scoped to the
SYSTEM'S OWNER rather than the caller. A caller reading through a shared project
owns no link in the chain, so the caller-scoped version would have handed them
an empty forest and truncated the cascade to a single system — a page rendering
with plausible wrong values and no error anywhere. The ACL already grants read
along the whole chain; this is the loading side keeping that promise, and it now
has a test naming the shared-project case.

`BASE_MODE` moves from the model to the cascade module, where it belongs: it is
a resolution rule, not a storage fact, and design_cascade.py deliberately
imports nothing so both access.py and the service can depend on it.

src/scribe/models/design_system.py

@@ -22,11 +22,6 @@ from sqlalchemy.orm import Mapped, mapped_column
 from scribe.models import Base
 from scribe.models.base import SoftDeleteMixin, TimestampMixin
 
-# The mode key that applies when no more specific one does. Emitted CSS puts it
-# on the base selector and every other key on a mode selector, mirroring how a
-# stylesheet is actually written: light on `:root`, dark layered over it.
-BASE_MODE = "base"
-
 
 class DesignSystem(Base, TimestampMixin, SoftDeleteMixin):
     __tablename__ = "design_systems"