style: rename `_params` and `sb` in generated code to clearer names

[pageton]

Summary

The SQL template renderer generates two local variables in every method body:

var sb strings.Builder
_params := make([]any, 0, N)

Two issues:

1. _params — The leading _ is the Go blank identifier. Using it as a prefix for an actively-used variable is misleading. It suggests the variable is discarded when it's actually central to the method.
2. sb — Too terse for a variable used across 20+ lines. A builder for a SQL query string deserves a clearer name.

Proposed

var buf strings.Builder
params := make([]any, 0, N)

Or alternatively query instead of buf since it's building a SQL query string.

Root Cause

• internal/gen/sqlparser.go — RenderSQLTemplate emits var sb strings.Builder and _params := make(...) literally
• Also referenced in internal/gen/template.go method body templates

Affected

All generated output. The hardcoded names in the SQL template renderer need updating, plus any test golden files that match these strings.