[2.x] perf(sticky): serve the excerpt as an attribute instead of including posts

[imorland]

Part 1 of the index firstPost work (context: discuss profiling — the ~110ms/23-query/123KB item). fof/synopsis and fof/gamification follow separately.

The problem

Sticky default-included firstPost on the discussions index to show a 175-char plain-text excerpt on stickied rows. Including a post serializes it in full: contentHtml renders through every extension's render callbacks, its visibility policies run (canEdit/canHide/canFlag — the PostPolicy chain), and the rendered HTML ships in the payload — for every discussion on every index view. Since sticky is bundled and enable_display_excerpt defaults to on, virtually every Flarum forum pays this.

The change

Stickied discussions carry a firstPostExcerpt attribute instead:

• Plain text via s9e\TextFormatter\Utils::removeFormatting() straight from the stored XML — no render pipeline, no callbacks, no policies. Capped at 200 chars (the frontend truncates to its display length).
• Visible only on stickied rows with the setting enabled.
• First posts load through the relationship buffer (EloquentBuffer::add/load, the same pattern as countRelation): one batched query for the stickied rows on a page, nothing for the rest, zero queries when the setting is off or no stickies are on the page.
• JS reads the attribute; the include push is gone. The server-rendered index document gets excerpts for free since attributes serialize with the discussion.

Why the buffer and not a constrained eager load — the part worth reviewing

My first version constrained an endpoint eagerLoadWhere to sticky rows' first posts. Measurement showed included posts dropping further than expected — because the constrained load marks firstPost as loaded (null) on every non-sticky row, and a pre-loaded relation short-circuits include resolution. Any client explicitly requesting include=firstPost for all discussions would silently get nulls — which is exactly what fof/synopsis ships today. The compat test (explicitly_requesting_first_posts_still_serializes_them_for_every_row) was written RED against that version and pins the contract.

Numbers

On a sticky-only install (integration tests): index payload goes from 20 serialized posts to zero; excerpt costs one batched query, none with the setting off.

On heavier installs the win is partially masked until other extensions stop forcing the include themselves — fof/geoip's addDefaultInclude(['firstPost.ipInfo']) still does (nothing on the list UI consumes it); follow-up PR queued there.

Testing

5 new integration tests, the payload-shape and attribute tests RED first: no posts serialized by default, excerpt only on stickied rows with formatting stripped, one batched query, zero queries with the setting off, and the explicit-include compatibility contract. Sticky suite 26/26, core discussions suite green, PHPStan clean. Verified live: excerpts render on stickied rows exactly as before.