fix: dbt build --empty compatibility via sv_ref/sv_source macros

[zaramzamzam]

Problem

dbt build --empty (used in CI/CD for schema validation) fails for semantic view models. The --empty flag rewrites ref()/source() into subqueries like (select * from TABLE where false limit 0), which breaks three contexts that require bare table identifiers:

1. CREATE SEMANTIC VIEW DDL — TABLES() clause
2. semantic_view() query function — table argument
3. get_ddl() calls in tests — string argument

Root cause

The --empty rewriting is hardcoded in Python (BaseRelation.render_limited() in dbt-core). There is no dispatchable macro to override it at the package level.

Solution

Two new utility macros: sv_ref() and sv_source().

They call dbt's native ref()/source() internally (preserving DAG lineage and execution ordering) but render the relation as DATABASE.SCHEMA.IDENTIFIER using the Relation object's component properties — which are stable public API and unaffected by --empty.

Usage

-- Semantic view definition (TABLES clause):
TABLES(t1 AS {{ dbt_semantic_view.sv_ref('my_table') }})

-- Querying a semantic view:
select * from semantic_view({{ dbt_semantic_view.sv_ref('my_view') }} metrics total_rows)

-- For sources:
TABLES(t2 AS {{ dbt_semantic_view.sv_source('my_source', 'my_table') }})

Standard ref()/source() should still be used in normal SELECT contexts where subquery wrapping is harmless.

Alternatives considered

Regex cleanup in materialization — Strip the --empty subquery pattern from compiled SQL before executing DDL. Rejected: tied to dbt's internal subquery format (an implementation detail), and silent regex mismatch is harder to debug than a clear error.

graph.nodes lookup (proposed in #11) — Manually resolve relation names from the dbt graph without calling ref(). Rejected: breaks DAG dependency tracking (dbt won't know about the relationship), and has correctness issues with custom schemas, cross-project refs, and aliases.

Tag-based exclusion — Exclude semantic view models from --empty runs. Rejected: fragile CI/CD configuration, provides zero validation value, and silently breaks when new models are added without the tag.

Verification

Both pass with 19/19:

dbt build --target snowflake --empty   # PASS=19 ERROR=0
dbt build --target snowflake           # PASS=19 ERROR=0

Closes #11