diff/build: change-reporting gaps #80

Found while working out the compatibility rules (tracking issue in core).

• history::load_schemas loads every tree entry, dep_* subtrees included, so comline diff on a package with dependencies treats a dependency as one of its schemas.
• comline diff reports no bump and no config changes. It only runs analyze_schema_changes, pairwise by index.
• comline build never prints config or dependency changes. A bump caused only by those prints "version X (no changes)".
• No machine-readable output for build or diff (--json exists only on targets --features).
• Nothing can fail a build on a breaking change. Is a --deny-breaking (or a CI-oriented comline diff --check) wanted?