feat(engine): add disabled_stages parameter to FusionEngine

[paint7pupper]

Summary

Adds a disabled_stages parameter to FusionEngine.__init__() so users can exclude specific pipeline stages entirely.

Motivation

When integrating claw-compactor into a coding agent, some stages add latency without helping — for example, log_crunch and search_crunch are irrelevant when only compressing source code. Being able to disable them at construction time is cleaner than modifying should_apply() logic.

Usage

# Skip dedup and abbreviation for code-only compression
engine = FusionEngine(disabled_stages={"semantic_dedup", "abbrev"})

# Minimal pipeline: only structural stages
engine = FusionEngine(disabled_stages={
    "photon", "semantic_dedup", "ionizer", "log_crunch",
    "search_crunch", "nexus", "abbrev"
})

Changes

• _build_pipeline() accepts disabled_stages: set[str] and filters stages by name
• FusionEngine.__init__() forwards the parameter
• Docstring updated with all valid stage names
• No behavior change when disabled_stages is not provided