Load: a flood of fake servers; unique-key lookups no longer scan

tools/pasture/flood/flood.cs answers as twenty fake servers (flood1..20.test)
and sends signed Creates, Likes and Follows at a set rate; load.sh measures
the answers, the queue's wait and processing times, its drain and a persona's
home timeline meanwhile (docs/LOAD.md has the method and the runs).

What the runs found:
- Every unique index was partial on $type: "string", which MongoDB never uses
  for an equality lookup, so every post by ObjectURI, actor by ActorURI,
  deleted object, domain block, remote instance and the rest was a
  collection scan (280 ms a post lookup at 30 000 posts). They are partial on
  $gt: "" now, which an equality on a string implies; MongoDB.Entities
  rebuilds them in place at the next start.
- Two inbox workers capped intake near 110 activities a second:
  Federation:InboxConcurrency and DeliveryConcurrency (default 8) set them.
- The indexes the plan listed as missing: a post's boosts and replies, a
  persona's boosts, who follows an actor, timeline rows by author, a post's
  likes and pins.

At 300 activities a second (200 let through, the rest 429 by the per-origin
limit) the queue wait went from 29 s to 6 ms at p50; with the limits lifted
PrivaPub processes about 900 a second, each in under 10 ms, and the home
timeline stays under 20 ms.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

CLAUDE.md

@@ -553,6 +553,10 @@ tools/pasture/run.sh down                                            # removes e
   `connection`) and its API is v3 (`/api/v3`, `sort=New`, `resolve_object` answering views). It takes private messages
   only as `ChatMessage`: a reply goes out as one, a first message cannot (G-0008). `scenarios/lemmy19.sh`, 29 checks
   and that gap.
+- **Load (`load.sh`, needs the `flood` peer):** `flood` (`flood/flood.cs`, published once into `.flood`) answers as
+  twenty fake servers and sends signed activities at a set rate; `load.sh --rate=N --seconds=N` measures the answers,
+  the queue's wait and processing times, its drain, and a persona's home timeline meanwhile, and keeps each run in
+  `out/load/`. `docs/LOAD.md` has the method and the runs.
 - **Account moves (`scenarios/moves.sh`, needs gts):** two fresh GoToSocial accounts made by its admin CLI; alice follows
   the old one, the new one names it as an alias (`/api/v1/accounts/alias`), the old one moves (`/api/v1/accounts/move`),
   and PrivaPub shows it `moved` while alice's follow stays. 6 checks.