Add PostgreSQL DurableStore substrate for WP-03

Introduce remora-durable with pool/migrations, a RemoraApiMutation-shaped
ledger, claim-fenced outbox projection, CAS helpers, and keyset cursors,
plus a kind Job acceptance suite against the hermetic fixture.

crates/remora-durable/migrations/0001_substrate.sql

@@ -0,0 +1,38 @@
+-- DurableStore substrate tables owned by remora-durable.
+-- Feature packages add their own migrations; they do not alter these tables.
+
+CREATE TABLE mutation_records (
+    scope_subject TEXT NOT NULL,
+    scope_operation TEXT NOT NULL,
+    operation_key TEXT NOT NULL,
+    kind TEXT NOT NULL,
+    -- Opaque RemoraApiMutation eligibility / authority binding (attempt, fence,
+    -- worker incarnation, or other claim material). Compared for exact replay.
+    eligibility_claim BYTEA NOT NULL,
+    request_fingerprint BYTEA NOT NULL CHECK (octet_length(request_fingerprint) = 32),
+    response_status TEXT NOT NULL CHECK (response_status IN ('accepted', 'rejected')),
+    response_body JSONB NOT NULL,
+    effect_applied BOOLEAN NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (scope_subject, scope_operation, operation_key),
+    CHECK (effect_applied = (response_status = 'accepted'))
+);
+
+CREATE TABLE outbox_messages (
+    id BIGSERIAL PRIMARY KEY,
+    topic TEXT NOT NULL,
+    payload JSONB NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    available_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    claimed_by TEXT,
+    claimed_until TIMESTAMPTZ,
+    -- Monotonic per-row claim epoch; with claimed_by forms the ownership token.
+    claim_generation BIGINT NOT NULL DEFAULT 0,
+    projected_at TIMESTAMPTZ,
+    attempt_count INTEGER NOT NULL DEFAULT 0,
+    last_error TEXT
+);
+
+CREATE INDEX outbox_messages_claimable_idx
+    ON outbox_messages (available_at, id)
+    WHERE projected_at IS NULL;