[Killerjunior]

Description

Closes #1745

migrateV1ToV2 previously only set schemaVersion when the existing value was not a number. An explicit schemaVersion: 1 was therefore left unchanged, even though the result was typed as SessionDataV2.

This change upgrades unversioned, V0, and explicit V1 sessions to V2; rejects invalid shapes and unsupported future versions with a discriminated result; validates Stellar addresses and timestamp ranges; and records migration attempts and failures. Unknown fields are preserved during upgrades.

The result type now distinguishes success from failure:

type MigrationResult =
  | { ok: true; data: SessionDataV2; migrated: boolean; fromVersion: number; toVersion: number }
  | {
      ok: false;
      reason: "INVALID_SHAPE" | "UNSUPPORTED_FUTURE_VERSION" | "UNSUPPORTED_LEGACY_VERSION";
      fromVersion: number;
    };

Tests were updated to import the module by relative path, cover explicit V0/V1 and future versions, invalid addresses and timestamps, stats, and generated idempotency cases. Fixtures with equal timestamps are now expected to be invalid because expiresAt must be greater than createdAt.