WP-O1: rename authorship root in PE-client mock, add legacy seed snapshot

Part of the origin-dimension implementation plan: adobe/serenity-docs#46 (branch `feat/origin_source`).

Package `packages/spacecat-shared-project-engine-client`. Own PR + `feat:` minor (GHCR mock image versioning matters — api-service resolves it with no fallback).

- `mock/seeds.js:74-79` `DIMENSION_ROOTS`: `source`→`origin`; `:84` `SOURCE_VALUES`→`ORIGIN_VALUES`; `:89` `SOURCE_ROOT_TAG_ID`→`ORIGIN_ROOT_TAG_ID`; `:222-238` tree.
- `:387-417` `SEED_IDS`: `sourceHumanTagId`→`originHumanTagId` — ids derive from `(parent, name)`, so every child id under the renamed root changes; api-service pins some as a cross-repo contract.
- Keep the cross-dimension collision fixture (`:98` `CATEGORY_CHILD_COLLIDING_NAME='human'`) — rename its prose, not its point.
- Add a legacy pre-rename seed snapshot (populated root named `source` with `ai`/`human` beneath) — needed for the tolerant-resolver gate (spec gate 3). Deleted by WP-O6.
- Update `docs/mock-usage.md:155-195` and route headers.
- Update pinned suites: `seeds.test.js`, `tag-id.test.js`, `tag-view.test.js`, `project-engine-mock.e2e.js`.

Exit: `npm run test:e2e` green; GHCR mock image manifest resolves.

Blocked on the tag-vs-metadata product decision.

Jira: SITES-48000

Start with mock/seeds.js, especially DIMENSION_ROOTS, the seed ID definitions, and the tree, then inspect docs/mock-usage.md and the route headers. Run the pinned seeds.test.js, tag-id.test.js, tag-view.test.js, and project-engine-mock.e2e.js suites; done means npm run test:e2e is green and the GHCR mock image manifest resolves, after the tag-vs-metadata decision is settled.

Written by the indexing model from the issue text.