Reduce dependency footprint with feature-gated DuckDB and compression

[Copilot]

Description

This PR addresses the reported dependency bloat for consumers that only need SQL splitting/parsing. It introduces feature gates so heavy runtime/build dependencies are opt-in, while preserving current behavior for default builds.

Changes

• Feature gating for heavy deps

  • Added Cargo features:
    • default = ["duckdb-query", "compression"]
    • duckdb-query (enables DuckDB + query command)
    • compression (enables gzip/bzip2/xz/zstd)
  • Marked duckdb, flate2, bzip2, xz2, zstd as optional.

• Compile-time gating in code paths

  • Gated DuckDB module exports/imports and query command wiring behind duckdb-query.
  • Kept existing default CLI/library behavior unchanged by retaining both features in default.

• Runtime behavior for no-compression builds

  • When compression is disabled, compressed extensions still detect as compressed but return a clear unsupported error instead of requiring codec deps.

• Docs and focused test coverage

  • Added README section showing default-features = false and optional feature selection.
  • Updated splitter unit tests for feature-aware behavior (compression-enabled and compression-disabled cases).

[features]
default = ["duckdb-query", "compression"]
duckdb-query = ["dep:duckdb", "compression"]
compression = ["dep:flate2", "dep:bzip2", "dep:xz2", "dep:zstd"]

Checklist

• Tests pass (cargo test)
• Code is formatted (cargo fmt)
• Linting passes (cargo clippy)
• Documentation updated (if needed)

Related Issues