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

## Summary

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

```go
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

```go
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.