[kvlonge]

Why

A target without connection = silently used an empty in-memory connection, so a build failed with "Schema dev does not exist", both when the project defined exactly one connection and when it defined none. Source freshness also required a strategy and type that the source already implies.

Changes

• A target that sets no connection uses the only named connection when there is exactly one and no top-level [connection] block (across sqlbuild_project.toml and sqlbuild_local.toml). With several named connections it fails at config load with D001, naming the target and the connections.
• With no connection configured at all, warehouse commands (build, plan, test, query, debug) fail with D001 before connecting; compile, format, lineage, scope, dag, rename and mv keep working. An explicit database = ":memory:" still works. Tests that relied on the empty fallback now set a connection.
• Freshness strategy is inferred from the keys present (column, query, or neither for adapter), and the column strategy takes type from the declared column when it maps clearly. Contradictions and unmappable types are errors; explicit settings keep working.
• Docs: default_target listed as optional, the default-connection rule, and the freshness inference table.

Verification

• Unit tests for target resolution, config-load validation and freshness parsing; an integration test for a real build into the only connection, sqb debug, the D001 failures and inferred freshness.
• Focused pytest (487 passed), ruff, fensu check and make check-ci.