loom-before-after

# Before/After Verification ## Overview A **delta-proof** proves a stage *caused* a change, not merely that the end state is valid. Capture what is true BEFORE implementation and what must be true AFTER; the pair distinguishes "my stage made it work" from "it already worked" (and "my fix resolved it" from "already fixed"). Without it, a green `acceptance` can pass on code that was already correct — proving nothing. > ⚠️ **`truths` is GONE** as a standalone field. Behavioral "after state" commands now go in **`acceptance`** (Simple string, or Extended object for output matching). The explicit, automated delta-proof mechanism is the **`before_stage`** / **`after_stage`** fields — unchanged, still `Vec<TruthCheck>`. A top-level `truths:` block is rejected as an unknown field. ## The delta pattern | Case | BEFORE (pre-condition) | AFTER (post-condition) | | ---- | ---------------------- | ---------------------- | | New feature | reproducer FAILS (feature absent) | reproducer SUCCEEDS | | Bug fix (inverted!) | reproducer SUCCEEDS (bug present) | reproducer FAILS (bug gone) | | Behavior change | old behavior observed | new behavior observed | For a bug fix the direction inverts: the reproducer "passing" means the bug is still there. `before_stage` with `exit_code: 1` on the reproducer, `after_stage` with `exit_code: 0`. ## `before_stage` / `after_stage` — the automated delta-proof (how loom runs them) Both are `Vec<TruthCheck>`; the plan author writes them. Exact lifecycle:...