[lovasoa]

Repeated variables now share a prepared parameter on native SQL Server, PostgreSQL, and SQLite connections. The SQL Server reproducer uses @p1 in both SELECT and GROUP BY and returns (0, 2), (1, 2), and (2, 2).

Reuse is scoped to one statement and keyed by the variable's name and lookup source. Function calls remain independent, and MySQL/ODBC positional placeholders keep one argument per occurrence. Regression coverage checks generated SQL, binding order, distinct lookup sources, SET changes including NULL, and live SQL Server grouping. Updated the unreleased changelog.

Chart documentation browser tests now wait for all charts to initialize before checking their output. The loading assertion checks every placeholder, avoiding an ambiguous multi-element locator.

Validation:

• Full cargo test on SQLite, SQL Server, PostgreSQL, and MySQL: 248 unit tests and 87 integration tests passed on each.
• All 148 Playwright tests passed locally using the CI-built Linux binary.
• npm test (82 tests), npm run build
• cargo fmt --all --check
• cargo clippy --all-targets --all-features -- -D warnings

Fixes #1558.