links: bind a [[link]] to a note, not to a string

A wiki-link was stored only as normalized TEXT, so a note's NAME was the edge.
Renaming it broke every inbound link — and the fix that shipped for that
(task 1848, option b) was `_rename_inbound_links`: rewrite the `[[Old Name]]`
text inside the body of every note that linked to the renamed one.

That works while an explicit title exists to hold still. It stops being
defensible the moment a note's name is just its first body line, which is where
M13 is going: fixing a typo in your opening sentence would silently edit other
notes' words, with nothing to opt out to. So this lands first, before the title
comes out, and that window never ships.

`note_links` gains `target_id`, bound when the link is written. `target_norm`
stays and is what an UNRESOLVED link carries — linking to a note that doesn't
exist yet is a supported way to create one, so a link has to be able to name a
target that isn't there. Resolution reads the id, falling back to the name only
where nothing was bound, which is what lets a forward link connect the moment
its target appears. `_claim_unresolved_links` then binds it, so the fallback is
a transitional state rather than a permanent one.

`_rename_inbound_links` and `rewrite_link_title` are gone. What replaced them
touches link rows only: a note's text is never modified by something happening
to a different note.

The client can no longer resolve links for itself, and that is the point. It
used to look `[[text]]` up in a client-side name index, which only held together
BECAUSE renaming rewrote the text everywhere. Now the written text can name
something the target is no longer called, and only the server holds the binding
— so each note serializes its resolved links (`norm`, `id`, and the target's
name as it stands NOW). A renamed note reads correctly everywhere it is linked
from, without a single body having been edited. Unresolved links are simply
absent and fall through to the create-on-click affordance that already existed;
so does the offline desktop store, which derives links at query time and has no
binding to send.

The name-fallback join is owner-scoped everywhere it appears. Bound ids were
resolved owner-scoped when written, but matching on display_title alone would
have let two users who each have a note called "Groceries" see the other's id
and name through an unresolved link (rule 47).

The new behaviour is all SQL and this suite runs without a database, so the
dead helpers' tests are removed rather than replaced. This repo has no
integration lane to hold that ground — noted, not papered over.

src/thoughtsync/models/note_link.py

@@ -10,14 +10,38 @@ from . import Base
 
 
 class NoteLink(Base):
-    """A [[wiki-link]] from a source note to a target title (normalized). Resolved
-    to a target note by matching target_norm against lower(trim(note.title))."""
+    """A [[wiki-link]] from a source note to another note.
+
+    Two target columns, and the pair is the point:
+
+    - ``target_id`` — the note this link actually points at, bound when the link was
+      written. This is what makes a link survive its target being RENAMED. A note's
+      name is derived from its first body line, so without an id the name is the
+      edge, and editing that line would silently break every inbound link (or, in the
+      older design, force a rewrite of every linking note's body).
+    - ``target_norm`` — the normalized link text, always stored. It is what an
+      UNRESOLVED link carries: `[[a note that doesn't exist yet]]` is a supported way
+      to create one, so a link has to be able to name a target that isn't there.
+
+    Resolution reads the id first and falls back to matching the norm against
+    ``notes.display_title``, which is how a forward link connects the moment its
+    target appears. ``_claim_unresolved_links`` then binds the id, so the fallback is
+    a transitional state rather than a permanent one.
+    """
 
     __tablename__ = "note_links"
-    __table_args__ = (Index("ix_note_links_target", "target_norm"),)
+    __table_args__ = (
+        Index("ix_note_links_target", "target_norm"),
+        Index("ix_note_links_target_id", "target_id"),
+    )
 
     id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
     source_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True), ForeignKey("notes.id", ondelete="CASCADE"), nullable=False
     )
+    # SET NULL rather than CASCADE: deleting the target un-resolves the link, it does
+    # not delete it. The link text is still sitting in the source's body.
+    target_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("notes.id", ondelete="SET NULL"), nullable=True
+    )
     target_norm: Mapped[str] = mapped_column(Text(), nullable=False)