(tasks) add tasks for harness

tasks/T2.4-schema-evolution-end-to-end.md

@@ -0,0 +1,88 @@
+# T2.4 — Schema evolution end-to-end
+
+| Field | Value |
+|---|---|
+| Phase | P2 — Durability hard parts |
+| Size | M — 1 to 3 days |
+| Status | Not started |
+| Flags | — |
+| Spec | inlined below |
+| Blocks | — |
+
+## Goal
+
+Change the event enum for real and prove a log recorded in P1 still folds. This
+is T0.4's framework exercised against actual history rather than a synthetic
+fixture.
+
+## Facts (inlined — no spec read needed)
+
+- Depends on T0.4 (upcaster registry) and T0.3 (`SchemaVersion` on every record).
+- The rules being tested: variants are **never removed or repurposed**;
+  deprecated variants stay decodable forever; migrations are upcasters applied on
+  read, never a rewrite of stored records.
+- The failure this prevents: two years of records, one `WorkEvent` variant
+  renamed, and the drop-and-re-fold property is silently gone. Nothing fails at
+  the time of the rename — it fails at the first cold rebuild.
+- The v1 fixture must be a **real P1 log**, not a hand-written one. A synthetic
+  fixture only tests the variants whoever wrote it remembered.
+
+## Steps
+
+1. Capture a full P1 run's log as a committed fixture, at v1, with its blobs.
+   Store the expected folded state alongside it.
+2. Make a genuine v2 change: add a field to an existing variant, or split one
+   variant into two. Do not rename or delete anything.
+3. Bump `CURRENT_SCHEMA` to v2 and register the v1→v2 upcaster.
+4. Run the fold over the v1 fixture under the v2 binary; assert the folded state
+   equals the recorded expectation.
+5. Record a fresh v2 fixture from the same workflow and assert the two folded
+   states agree — an upcast v1 log and a native v2 log must be
+   indistinguishable downstream.
+6. Extend T0.3's directory-walk fixture test so both version folders are covered
+   permanently, not just in this task's test.
+
+## Acceptance
+
+- A v1 log fixture recorded in P1 still folds correctly after the event enum
+  changes.
+
+## Verify
+
+**Harness:** a **real P1 run's** log committed as a fixture, with its expected
+folded state committed beside it. Not hand-written.
+
+**Integration test** — `tests/it_schema_evolution.rs`:
+1. Under the v2 binary, fold the committed v1 fixture → `state_a`.
+2. Assert `state_a` equals the committed expected state, byte for byte.
+3. Record a fresh v2 run of the same workflow → fold → `state_b`.
+4. Assert `state_a` and `state_b` are equivalent under the fields that should not
+   have changed. An upcast v1 log and a native v2 log must be indistinguishable
+   downstream.
+5. Assert the v1 fixture file's bytes are unchanged by the test run — the fixture
+   is read-only history.
+6. Variant-removal guard: a test that enumerates v1's variant set and asserts
+   every one is still decodable under v2.
+
+**Command:** `cargo test -p log schema_evolution`
+
+**False pass:**
+- Regenerating the fixture when the test goes red. That is the one move that
+  destroys the guarantee — the fixture is the historical record, the upcaster is
+  what changes. Make the fixture path read-only in the test and assert on that.
+- A v2 change that touches no variant present in the P1 fixture, so no upcaster
+  logic runs. Step 6 catches the removal case; choose a v2 change that provably
+  affects a recorded variant.
+
+## Traps
+
+- "Fixing" the fixture bytes when the test fails. The fixture is the historical
+  record; the upcaster is what changes.
+- A v2 change that removes a variant, which passes this test today because
+  nothing in the P1 fixture used it, and breaks the moment an older log does.
+
+---
+
+Background (not required to do this task):
+[rust-agentic-sys.md](../../../rust-agentic-sys.md) §8.7 ·
+[rust-agentic-task.md](../../../rust-agentic-task.md)