getSeoMeta defaults: implement suggestion 2 from #1518

[swissky]

Issue #1518 (milestoned 1.0) describes the SEO-panel footgun and lists three suggestions. Suggestion 1 (docs gotcha) is covered by the reporter's PR #1519 and our #1900. This discussion tracks suggestion 2, the ergonomic helper:

let getSeoMeta accept defaultTitle/defaultDescription. Today its only fallbacks are data.title/data.excerpt, so authors with computed titles (e.g. "Maeta — {title} (cover of {artist})") must drop down to getContentSeo and merge panel-vs-default by hand.

Proposed design (implementation ready)

Two new optional fields on SeoMetaOptions:

• defaultTitle — priority: SEO panel title > defaultTitle > data.title
• defaultDescription — priority: SEO panel description > defaultDescription > data.excerpt

Editor-set panel values always win, so the panel stays authoritative; the defaults only replace the hard-coded data.title/data.excerpt fallbacks. Purely additive — existing callers are unaffected. The siteTitle suffix applies to the default title like any other page title.

This removes the last reason to hand-roll <title> on a page with computed titles, which is exactly the pattern that bypasses the panel (including noindex) silently.

Suggestion 3 (dev-mode warning when data.seo is set but never read) is intentionally not part of this: it needs render-path tracking state and seems better discussed separately if the docs + helper don't defuse the footgun enough.

PR incoming, linked here.

[swissky]

PR is up: #1957