chore(spec): regenerate stale ADR-0087 artifacts (spec-changes.json + upgrade guide) (#3330)

* chore(spec): regenerate spec-changes.json for the 15→16 registry updates

`spec-changes.json` is a generated artifact (ADR-0087 D4) — a pure projection
of the D2 conversion table + D3 migration chain, enforced in CI by
`check:spec-changes` (the "Check Generated Artifacts" gate).

The committed manifest went stale on `main`: the dashboard-widget `.strict()`
migration (`dashboard-widget-strict-unknown-keys`, changeset
`dashboard-widget-strict-3251`) landed and moved the protocol major to 16, but
the manifest was never regenerated. As a result `check:spec-changes` fails on
every open PR, not just this one.

Regenerated with `pnpm --filter @objectstack/spec gen:spec-changes`. Pure
re-projection of already-merged registries — no source or behaviour change:

  - protocolVersion 15.0.0 → 16.0.0
  - aggregate to: 15 → 16
  - new 15→16 migrated record: dashboard-widget-strict-unknown-keys

No new changeset: the underlying migration already shipped its own
(`dashboard-widget-strict-3251`); this is a generated-artifact re-sync only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011iJLjqToxNv1aYP3syQtRp

* chore(spec): regenerate protocol-upgrade-guide.md for the 15→16 registry updates

Second generated artifact stale from the same ADR-0087 registry change as the
`spec-changes.json` re-sync in this PR. The "Check Generated Artifacts" job runs
`check:spec-changes` then `check:upgrade-guide`; with the manifest fixed, the
upgrade guide surfaced as the next stale projection (the job had been failing
fast on the manifest before).

Regenerated with `pnpm --filter @objectstack/spec gen:upgrade-guide`. Pure
re-projection of already-merged registries — no source change:

  - Current protocol 15.0.0 → 16.0.0
  - new "Protocol 15 → 16" section: dashboard-widget-strict-unknown-keys

`check:skill-docs`, `check:spec-changes`, and `check:upgrade-guide` all pass
locally.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011iJLjqToxNv1aYP3syQtRp

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-zhuang