paid-media: missing `title` on nested leaf properties in 3 fieldgroup schemas

## Problem

Several nested-object leaf properties in the XDM paid-media fieldgroup schemas are missing the `title` keyword (and `description` in most cases). When these schemas are materialized into a CJA dataview, the affected components fall back to displaying the full dot-notation `schemaPath` as their name (e.g. `paidMedia.attributionMetrics.crossDeviceAttribution.devicePathBreakdown.mobileToMobileApp`) instead of a human-readable label.

Sibling properties at the same nesting level all have `title` set, so this looks like an oversight in 3 inline object definitions rather than an intentional pattern.

## Detection

Spotted while comparing a CJA paid-media dataview against the source schemas: 41 components in that dataview have `name == schemaPath`, and all 41 fall into the 3 groups listed below.

## Affected fields

**`components/fieldgroups/paid-media/core-paid-media-attribution-metrics.schema.json`**

- `xdm:devicePathBreakdown` leaf properties — `mobileToDesktop`, `desktopToMobile`, `mobileToMobileApp` (no `title`, no `description`)
- `xdm:channelSequence.items` properties — `channel`, `position`, `weight` (no `title`, no `description`)

**`components/fieldgroups/paid-media/core-paid-media-campaign-details.schema.json`**

- `xdm:dayParting.schedule.items` properties — `dayOfWeek`, `startHour`, `endHour` (no `title`, no `description`)

## Fix

Add `title` and (where useful) `description` to each of the above leaf properties, matching the style and tone of the sibling fields already present. Per [CONTRIBUTING.md](https://github.com/adobe/xdm/blob/master/CONTRIBUTING.md#coding-styleguides): *"Provide a description and title for each schema and each property"*.

No breaking changes — the wire-format property names are unchanged; only `title`/`description` metadata is added.

Start with the three affected inline object definitions in components/fieldgroups/paid-media/core-paid-media-attribution-metrics.schema.json and components/fieldgroups/paid-media/core-paid-media-campaign-details.schema.json, comparing each leaf with its documented siblings. Add human-readable title and useful description metadata to the listed properties, preserve their wire-format names, and verify that all affected leaves now have the required metadata.

Written by the indexing model from the issue text.