feat(poet): add VarDecl and convert query functions to structured statements

Add VarDecl statement type to poet package for variable declarations
inside function bodies (e.g., "var items []Type").

Convert all standard SQL driver query functions to use structured
poet statements instead of strings.Builder:
- addQueryOneStd: uses Assign, VarDecl, If, Return
- addQueryManyStd: uses Assign, VarDecl, Defer, For, If, Return
- addQueryExecRowsStd: uses Assign, If, Return
- addQueryExecLastIDStd: uses Assign, If, Return
- addQueryExecResultStd: uses Assign, If, Return

Slice queries (sqlc.slice) fall back to RawStmt due to their
complex dynamic SQL generation requirements.

Added wrapErrorReturn helper for consistent error wrapping logic.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: claude