docs: re-measure the corpus-access table against the published corpus (#56)

figures predate the row-group rechunk (cog_pipeline#93, published 2026-08-09)
and reported the mirrored column as 'local speed' with no number -- hiding the
largest difference available to a user.

Measured 2026-08-10, fresh R session per arm, against the live corpus at
pipeline_commit 3d28ddd. Madison WI, 16-core Linux workstation.

Three findings the old table could not express:

- A local mirror is 60-80x faster. A one-off question is ~12 s end to end
  remotely against ~0.15 s mirrored. Stated outright now, because it is a
  bigger and cheaper win for users than anything in the R code.

- Opening the session is the LARGEST remote cost (~7.5 s), bigger than any
  individual query, and it lands on the user's first query rather than on
  library(). The old table accounted for it nowhere, so every per-query figure
  was quietly missing it.

- The remote cost is round-trips, not scanning: a repeat query over
  already-touched partitions is ~1.5 s against ~4 s cold, and a full-history
  query costs ~7 s whether it runs first or last (verified by running the arms
  in both orders). This is why #93's 1.4-1.7x, measured through cog-api against
  a local mount, does not show up on the remote path -- there, network latency
  swamps scan time.

Corpus size corrected to ~201 MB: row-group chunking added ~3.4%, and 190.6 was
ambiguous between MB and MiB besides. Measured from the manifest and on disk.
The 0.3.0 NEWS section keeps 190.6 -- it was correct for that release.

Also documented HTTP 429: a burst of remote queries gets rate-limited by the
host. Hit while taking these measurements.

NEWS.md

@@ -41,6 +41,30 @@ Two refusals rather than silent surprises:
   per requested name with a sidecar covering all of them; a page of that is not
   a page of anything the caller asked for.
 
+## Documentation: the corpus-access table is re-measured and honest
+
+The README's "two ways to read the corpus" table carried figures taken before
+the corpus was re-chunked into row groups (cog_pipeline#93, published
+2026-08-09) and reported the mirrored column as "local speed" with no number at
+all. Re-measured 2026-08-10 against the published corpus (`pipeline_commit
+3d28ddd`), fresh R session per arm:
+
+* **A local mirror is roughly 60-80x faster.** A one-off question costs ~12 s
+  end to end remotely against ~0.15 s mirrored. That is the largest single
+  difference available to a user and it is now stated outright rather than left
+  as "local speed".
+* **Opening the session is the largest remote cost** (~7.5 s -- manifest fetch
+  plus 23 view registrations over HTTPS), larger than any individual query, and
+  it lands on the first query rather than on `library(uscogdata)`. The old table
+  did not account for it anywhere.
+* **The remote cost is round-trips, not scanning.** A repeat query over
+  already-touched partitions is ~1.5 s against ~4 s cold, and a full-history
+  query costs ~7 s whether it runs first or last.
+* The corpus size is **~201 MB**, not 190.6 MB -- row-group chunking added ~3.4%
+  and the old figure was ambiguous between MB and MiB besides.
+* Documented that a burst of remote queries can be rate-limited by the host
+  (`HTTP 429`), which is another reason to mirror for real work.
+
 ## Cohorts can be named by predicate, not just by id
 
 `cog_spending()`, `cog_revenue()` and `cog_balances()` gain optional `state`