Fix XSD path for installed package (v0.3.1)

Use __file__-based path resolution for envelope.xsd so the schema
loads correctly when xml-pipeline is installed via pip.

Also:
- Add build artifacts to .gitignore
- Bump version to 0.3.1

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -9,6 +9,11 @@ venv/
 .venv/
 ENV/
 
+# Build artifacts
+build/
+dist/
+*.egg-info/
+
 # Secrets & config
 .env
 .env.local

pyproject.toml

@@ -9,7 +9,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "xml-pipeline"
-version = "0.3.0"
+version = "0.3.1"
 description = "Schema-driven XML message bus for multi-agent systems"
 readme = "README.md"
 requires-python = ">=3.11"

xml_pipeline/__init__.py

@@ -2,4 +2,4 @@
 xml-pipeline: Tamper-proof nervous system for multi-agent organisms.
 """
 
-__version__ = "0.3.0"
+__version__ = "0.3.1"

xml_pipeline/message_bus/steps/envelope_validation.py

@@ -12,12 +12,16 @@ for diagnostic <huh>).
 Part of AgentServer v2.1 message pump.
 """
 
+from pathlib import Path
+
 from lxml import etree
+
 from xml_pipeline.message_bus.message_state import MessageState
 
 # Load envelope.xsd once at module import (startup time)
-# In real implementation, move this to a config loader or bus init
-_ENVELOPE_XSD = etree.XMLSchema(file="xml_pipeline/schema/envelope.xsd")
+# Path is relative to this file: steps/ → message_bus/ → xml_pipeline/ → schema/
+_SCHEMA_PATH = Path(__file__).parent.parent.parent / "schema" / "envelope.xsd"
+_ENVELOPE_XSD = etree.XMLSchema(file=str(_SCHEMA_PATH))
 
 
 async def envelope_validation_step(state: MessageState) -> MessageState: