feat(durable): scaffold zeph-durable crate, core newtypes, journal schema

[bug-ops]

Summary

Scaffolds the new Layer-0 zeph-durable crate (spec-064) — the foundation of the native durable
execution layer. This first slice (C1) is type-level only: it establishes the crate skeleton,
core newtypes, the journal data model, the effect contract, pure-data config, the error type, and
the persistence schema. No execution behavior exists yet — the journal writer, backends, replay
cursor, and the durable step primitive land in follow-up issues of epic #4707.

What's included

New crate crates/zeph-durable/:

• ids.rs — ExecutionId / PromiseId / TimerId (UUIDv7), StepId, JournalSeq,
  IdempotencyKey, and the ExecutionKind discriminator. Private fields, smart constructors,
  serde-round-trip stable. IdempotencyKey::derive uses BLAKE3 derive_key with a domain-separation
  context string and length-delimited (injective) input.
• journal.rs — the Journal trait (native async via RPITIT + Send, matching the
  LlmProvider/ToolExecutor convention), JournalEntry, the closed EntryKind enum, and
  ExecutionStatus.
• effect.rs — EffectClass (the bare contract needed by EntryKind; OnAmbiguous /
  EffectIntentSubClass and the construction-time policy land in C4).
• config.rs — pure-data DurableConfig + RetentionPolicy with container-level #[serde(default)]
  mirroring [durable] TOML, and a type-safe DurableBackend enum.
• error.rs — DurableError (#[non_exhaustive], metadata-only messages, INV-5).
• sealed.rs, lib.rs, README.md.

Schema (zeph-db): durable_executions / durable_journal / durable_promises /
durable_timers added as numbered migrations 097–100 in both
zeph-db/migrations/sqlite/ and .../postgres/ (matching file counts, schema-equivalent:
BLOB→BYTEA, AUTOINCREMENT→BIGSERIAL, epoch columns INTEGER→BIGINT), including the
UNIQUE partial index idx_durable_journal_result. zeph-durable owns no .sql and no
sqlx::migrate!.

Acceptance criteria

• INV-1 — no dependency on zeph-llm/zeph-memory/zeph-core/zeph-sanitizer or any
  business-layer crate (cargo tree -p zeph-durable → only zeph-db + zeph-common).
• INV-14 — four durable_* files in both dialect dirs (matching counts); no .sql / no
  sqlx::migrate! inside zeph-durable.
• IdempotencyKey::derive is domain-separated and injective; a boundary-shift test asserts
  distinct keys.
• Newtype smart constructors expose no public field; tests cover ExecutionId::new uniqueness
  and serde round-trip of every newtype.
• DurableConfig / RetentionPolicy serde defaults match the spec; an empty TOML table
  deserializes to every default.
• EntryKind is a closed enum (exhaustive match compiles); control entries carry no payload.
• cargo check/clippy -D warnings/cargo doc pass; every pub item is documented.

Verification

Check	Result
cargo +nightly fmt --check	clean
cargo clippy --workspace --all-targets -- -D warnings	pass
RUSTFLAGS=-D warnings cargo check --workspace --all-targets --features desktop,ide,server,chat,pdf,scheduler	pass
cargo nextest run --workspace --features full --lib --bins	11046 passed, 0 failed (21 new)
cargo test --doc --workspace --features desktop,ide,server,chat,pdf,scheduler	pass (12 new)
RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps -p zeph-durable	clean
SQLite migrations applied to fresh DB + full set via run_migrations	pass (4 tables, 5 indexes)

Out of scope (later issues)

PayloadCipher (C2/#4945); JournalWriter/backends/Journal impl (C3/#4946); DurableStep /
DurableContext and the ambiguity policy (C4/#4947); CLI / root-config wiring / TUI (C6/#4949).

Part of #4707.

Closes #4944.