test(core): persisted-schema round-trip fuzz + reject future checkpoint versions (#31)

[ZhiXiao-Lin]

Follow-up #31 (persisted-schema round-trip fuzz across versions).

What

• New core/tests/test_persisted_schema_roundtrip.rs — a dependency-free, seeded (xorshift) round-trip fuzz over the persisted graph: LoopCheckpoint, RunRecord (incl. all three 3.3.0 AgentEvent variants), SubagentTaskSnapshot, TraceEvent, VerificationReport, Message/ContentBlock, and the largest root SessionData. Asserts serialize→deserialize→re-serialize stability (stronger than PartialEq, which several of these don't derive). Plus cross-version coverage:

  • Backward-compat: pre-3.3.0 payloads missing schema_version / the identity fields (tenant_id/principal/agent_template_id/correlation_id) / using the legacy todos alias all load with correct defaults.
  • Forward-compat: unknown extra fields are ignored — the invariant that keeps rolling, mixed-version cluster upgrades safe (would break the day a persisted type gains #[serde(deny_unknown_fields)]).
  • The #[serde(untagged)] ToolResultContentField and the persisted untagged PermissionRule (bare-string vs {rule:...}) hazards.

• Fix: reject future checkpoint schema versions. LoopCheckpoint documented that loads from a future, incompatible schema version must be rejected, but no impl enforced it. Adds LoopCheckpoint::ensure_loadable(), called in both the file and memory store load paths — so resume_run surfaces the error and the live-run sink starts fresh rather than misinterpreting a checkpoint whose field semantics may have changed.

Verification

• 10/10 tests pass. The fuzz already earned its keep — it caught a real Some(serde_json::Value::Null) → None asymmetry in Option<Value> fields (a serde property, fixed in the generator, not a code defect).
• No regressions (loop_checkpoint 5/5, store 36/36). CI gates green locally: cargo fmt --all --check, cargo clippy --workspace --lib --bins -D warnings; new test is clippy-clean.
• Scope/coverage was hardened in response to an adversarial review (added SessionData + PermissionRule coverage that an initial pass missed).

Pointer-stable, additive; no public API change beyond the new LoopCheckpoint::ensure_loadable() method.