Sprint 5J: deterministic hybrid artifact merge

[samrusani]

Summary

• merge compile-path lexical and semantic artifact candidates into one deterministic artifact section
• preserve source provenance, deduplicate by durable chunk id, and trace merge decisions
• add unit and integration coverage and update build/review docs

Verification

• ./.venv/bin/python -m pytest tests/unit/test_compiler.py tests/unit/test_main.py tests/unit/test_response_generation.py
• ./.venv/bin/python -m pytest tests/unit
• ./.venv/bin/python -m pytest tests/integration/test_context_compile.py
• ./.venv/bin/python -m pytest tests/integration

Review

• REVIEW_REPORT.md verdict: PASS
• Control Tower decision: MERGE_APPROVED