fix(homepage): boot-seed HomepageShelves; retire seed CSV + manual seed:thirdparty

[jung-thomas]

Why

HomepageShelves kept losing its curated/third-party shelf links (Prior Labs, Dremio, Reltio, n8n, Iceberg, Kaggle, Vercel, Hugging Face…) — the recurring "shelf links disappeared" incidents.

Root cause (verified): while the seed CSV lived in db/data, cds build --production compiled it to an .hdbtabledata with include_filter: [], so HDI treated the table as fully tabledata-managed and full-replaced it on every deploy whose CSV hash changed — deleting runtime-seeded rows and admin edits. Commit 1404c4c4 moved the CSV to test/data (which stopped the wipe on PROD, since --production then emits no tabledata) but it never re-seeded the already-lost rows, and the manual npm run seed:thirdparty promotion was DEV/PROD-ambiguous (it followed the CDS binding, so it could silently target the wrong environment).

What

Replace both fragile mechanisms with an idempotent, non-destructive boot seed:

• srv/lib/homepage/homepage-shelves-defaults.js — single inline canonical baseline (74 SAP entries + 23 third-party links = 97 rows across all 7 verbs). Ships with the srv bundle; no runtime file-read / packaging risk.
• srv/lib/homepage/seed-homepage-shelves.js — insert-if-missing on (verb,url), called from cds.on('served') (srv/server.js, mirroring the existing ui-event-saved-queries / ai-grading seed precedents). Never overwrites admin edits, self-heals a deleted baseline row on next restart, fully non-fatal, runs in every profile (cds watch, unit tests, hybrid, production).
• Retired: test/data/…-HomepageShelves.csv, db/data/staging/homepage-thirdparty.json, scripts/seed-thirdparty.js + its two tests, and the seed:thirdparty npm script (now a helpful error stub pointing at the new path). This also removes the DEV/PROD-ambiguous manual-seed footgun.
• admin-managed-seed-csvs guard test now asserts the CSV is gone from both data dirs.
• Docs: new "HomepageShelves seeding" section in docs/developers/architecture/homepage.md.

Guarantees

• No full-replace ever — HomepageShelves stays admin-managed on HANA (structure-only HDI artifacts, no .hdbtabledata).
• Admin edits preserved — seed only inserts missing (verb,url) rows.
• Self-healing — a curated link that goes missing reappears on the next srv restart.

Testing

• New unit tests: test/unit/homepage/homepage-shelves-defaults.test.js (schema/enum/unique/persona-tag validation of the baseline) and test/unit/homepage/seed-homepage-shelves.test.js (boot-populates third-party, idempotent, non-destructive, self-heals).
• Existing homepage-seed, homepage-shelves-crud, homepage-link-health and the guard test stay green.
• Full unit suite: 9078 passed, 0 failures. (17 file-level failures are pre-existing fresh-worktree env gaps — missing frontend deps needing npm run setup, and the generated srv-qa/lib/parsers.bundle.mjs — none related to this change.)

Ops note

PROD HomepageShelves was manually re-seeded and a PROD content rebuild run to restore the links immediately (out of band). Once this ships, the baseline is self-healing on every boot; DEV — which still lacked the rows — will be seeded automatically on next deploy.