feat: improve native test diagnostics

[kvlonge]

Why

Native SQL tests exposed actionable-diagnostics gaps during large fixture migrations: missing columns failed one at a time in warehouses, collection type conflicts lacked fixture context, parameter cases could not run independently, and multi-model mock boundaries were difficult to inspect. Complex strings in VALUES also triggered a planner hang because expected CTEs were re-parsed with a nested-parenthesis regex.

Changes

• align changed column values when exactly one safely sampled row differs each way
• add sqb test --case NAME with parent/model selection-aware errors
• add offline sqb test --inspect for mocks, replacement boundaries, real chains, expected models, and unsatisfied leaves
• statically aggregate provable missing-column and collection/scalar fixture errors before connection
• keep ambiguous star projections and transformed types conservative
• carry expected CTEs structurally instead of regex re-parsing
• preserve complex comma/bracket/parenthesis strings in DuckDB execution and Snowflake compilation
• document all behavior and regenerate the packaged SQLBuild skill

Verification

• 60 focused unit, integration, and real CLI tests passed with xdist
• DuckDB execution and Snowflake-dialect compile fixture regressions
• exact cache-focused SQL test coverage passed
• uv sync --all-extras
• make check-ci
• Fensu: zero faults and generated skills current
• one bounded independent review plus focused follow-up
• public-repository hygiene scan and git diff --check
• docs PR docs: explain native test diagnostics sqlbuild-docs#44 merged