Add W024 select-distinct-suspicious (DISTINCT + JOIN band-aid heuristic)

[Pawansingh3889]

What

New warning rule W024 select-distinct-suspicious that fires when a single statement contains both top-level SELECT DISTINCT and any JOIN keyword.

Developers reach for SELECT DISTINCT to swallow row duplication caused by an over-wide JOIN cardinality, often because a join condition is missing or because the right tool was actually GROUP BY. The result reads more rows than necessary and hides the underlying join-condition bug. The pattern is common enough in real codebases that it earns its own rule rather than living inside S001.

What's flagged

SELECT DISTINCT c.id, c.name
FROM customers c
JOIN orders o ON c.id = o.customer_id;

What's not flagged

-- single-table DISTINCT, no JOIN cardinality blow-up to mask
SELECT DISTINCT country FROM customers;

-- aggregate-DISTINCT is a different pattern; regex anchors on SELECT directly preceding DISTINCT
SELECT COUNT(DISTINCT c.id) FROM customers c JOIN orders o ON c.id = o.customer_id;
SELECT SUM(DISTINCT amount) FROM payments p JOIN customers c ON p.cust_id = c.id;

Implementation notes

• multiline = True plus check_statement so DISTINCT and JOIN can sit on different lines of the same statement.
• • strip_strings_and_comments (lifted into base.py previously for E009) is reused so DISTINCT or JOIN tokens inside string literals or -- / /* */ comments cannot fire the rule.
• • The regex \bSELECT\s+DISTINCT\b only matches top-level usage. COUNT(DISTINCT ...), SUM(DISTINCT ...), etc. fail to match because the keyword preceding DISTINCT is a (, not SELECT.

Tests

11 new tests in tests/test_new_rules.py covering inner join, left join, multi-line, case-insensitive, single-table DISTINCT (passes), COUNT(DISTINCT) with JOIN (passes), SUM(DISTINCT) with JOIN (passes), JOIN without DISTINCT (passes), DISTINCT inside a string literal (passes), DISTINCT inside a -- comment (passes), and a message-content assertion. Registry counts bumped to 42 / 31 warnings. Full suite: 254 passed locally, 1 skipped.

Closes #40.