Refactor journal_append_generic() - Separate concerns

[flyingrobots]

Part of SRP Refactoring (Priority 1: Core Infrastructure)

Current Problem

journal_append_generic() in src/journal/writer.c mixes buffer management, encoding, and batching logic.

Refactoring Plan

Split into focused functions:

• prepare_journal_buffer() - Allocate and manage buffer
• encode_edges_batch() - CBOR encoding only
• should_commit_batch() - Batch size decision logic
• commit_journal_batch() - Commit creation only

Acceptance Criteria

• Original function replaced with orchestrator
• Buffer management separated from encoding
• Batching logic isolated and testable
• Each function has single responsibility
• Unit tests for each new function
• No regression in functionality

Technical Notes

• Consider extracting buffer management to separate module
• Make batch size configurable
• Ensure thread safety if needed

Related to SRP_REFACTORING_PLAN.md Priority 1.2