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

[byteclimber]

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