Theme: catch SQL patterns that compile fine and run slowly.

Builds on the v0.6 W017 / W018 / W019 trio (leading-wildcard-like, or-across-columns, count-distinct-unbounded).

In scope

New performance-flavoured warning rules:

• cross-join-without-on (W021 candidate)
• function-on-indexed-column (W022 candidate)
• scalar-udf-in-where (W023 candidate)
• negate-of-equality (W024 candidate)
• select-star-in-view-or-cte (W025 candidate)

Also in scope:

• Dialect-aware messaging on existing rules (Postgres / T-SQL / Redshift)
• --severity threshold flag improvements (comma-separated lists)
• Self-benchmark perf gate in CI (catch regressions in scan speed)

Out of scope for v0.7

LSP server, sqlglot-based AST detection, auto-fix application, plugin system. See ROADMAP.md for the full picture.

How to contribute

Each new rule mirrors the W019 shape. See PR #29 and PR #21 for full review cycles. Pick a good first issue tagged to this milestone, follow the rule-authoring template in CONTRIBUTING.md, and open a PR.