[datlechin]

Summary

• The PostgreSQL driver finds pgvector's vector, halfvec and sparsevec types on connect, and spells a column read from a table with its dimension count from PQfmod: vector(1536).
• New vector column type. The grid draws [0.0123, -0.0456, 0.0789, …] 1536 dims instead of the whole literal.
• Double-click, Return or the chevron opens a viewer: one element per row with its index, Copy as JSON, Edit as Text. The row inspector opens the same viewer.
• Save refuses a value whose length differs from its vector(n) column and names the column, before anything reaches the server. The viewer disables OK on a wrong length.
• An emptied vector cell stages NULL instead of '', which pgvector refuses.

Not in this PR

• vector(n), halfvec(n) and sparsevec(n) in the Structure type picker when the extension is installed: the picker has no source for extension base types without a new PluginKit API. Typing vector(1536) there works today.
• Fetching large values only when a cell opens: Fetch BLOB values only when a cell is opened #2490.

Test plan

• New: PgvectorTypesTests, VectorLiteralTests, ColumnTypeVectorTests, CellDisplayFormatterVectorTests, CellInteractionResolverVectorTests, FieldEditorResolverVectorTests, RowChangeStatementFactoryVectorTests. None compiles on main.
• Unit runs over the changed types: 439 passed, and 150 after the review fixes.
• Measured on pgvector 0.8.7, PostgreSQL 17: text forms, PQfmod 1536 on a table column, PQftable 0 on v::vector(2), the server's expected 3 dimensions, not 2, and the probe skipping a composite named vector.

Screenshots

Before, a vector cell drew its literal, [0.0989,0.0412,-0.0544,-0.1,…, to the column's edge. The old build would not launch under XCUITest here, so there is no before shot.

Fixes #3599