Measure k: the per-page script-driven origin calls the offload figure counts on neither side

[harper-joseph]

Follow-up to #152. The console states net origin offload documents-only and says the fifth term is
missing from both sides of the ledger: the XHR/API calls a page's own scripts make when a rendering
crawler runs it. This issue measures that term so the console can count it instead of stating it.

The caveat is still in the tree (packages/console/README.md): "None of it passes through the plugin,
so the figure is documents-only on both sides … the true net offload for rendering crawlers is higher
than shown."

Status — updated 2026-09-16

Not shipped. #154 implemented all three stages and was closed unmerged; nothing from it is on
main (no hydration_calls, no uncacheableSubrequests, no scriptsStripped, no rendersJs). The
work below stands as designed; only the version plan needed renumbering, because the tags it reserved
were consumed by the #155–#159 release train.

stage	was	now
browser	v1.22.0	v1.25.0 (main is 1.24.0)
plugin	v0.65.0	v0.68.0 (0.66.1 shipped #106; 0.67.0 is taken by #102's PR #162)
console	v0.13.0	v0.14.0 (main is 0.13.0 — went to #159)

One interaction to fold in before re-implementing: #158's endpoint switch drops the page's own
price/availability XHR from bot renders via a fleet block.urlPatterns entry. That shrinks the
renderer's own k on that deployment and raises the blocked count — which the design already
reports, and which is exactly why blocked is counted separately rather than silently lost.

The quantity

k(page) = same-origin subrequests a page load makes whose response no shared cache would serve —
the calls that reach the origin whoever runs the page. Then, per page-view by a crawler that executes
scripts:

	origin requests
without prerender	1 + k (document, then the page's calls)
cache-served, snapshot without scripts	0 — k is saved
cache-served, snapshot with scripts	k — incurred
proxied origin page	1 + k — incurred
our own render	1 + k — incurred (the renderer runs the page too)

Where each piece is measured

Browser (v1.25.0) — the renderer's response hook already sees every same-origin response and
already inspects cache headers for its own resource cache. Add a pure classifier over
(request, response) → uncacheable (explicit: non-GET, no-store/private/no-cache,
Set-Cookie, uncacheable status), cacheable (explicit positive freshness: s-maxage/max-age/
Expires), or unspecified (no freshness info — CDN-default dependent; reported, counted on neither
side). Also count same-origin requests our block list aborted (blocked), so the undercount is
visible. Carry the counts on the attempt and post them with the result as
subrequests: { sameOrigin, cacheable, uncacheable, unspecified, blocked }, plus
scriptsStripped: <postProcess.stripScripts> so the plugin knows whether the stored snapshot can
hydrate at all. Cost: string checks per response; ~5 ints on the wire.

Plugin (prerender-v0.68.0)

• PrerenderedPage gains uncacheableSubrequests: Int and scriptsStripped: Boolean (nullable; older
  rows read as unknown). Written on store.
• render gains series subrequests (method = kind) — a value per posted result, so Σ = mean × count;
  this is the render fleet's own k cost.
• Registry: analytics.bots[].rendersJs (boolean). Defaults true for the documented renderers only —
  Googlebot, Google InspectionTool, Bingbot, Applebot, YandexBot. Every AI crawler in the registry runs
  nothing and stays unflagged.
• Serve path: for a request from a flagged crawler, emit hydration_calls (new metric; it needs its own
  three slots): path = side (saved | incurred | unknown), method = bot, type = source, value = k.
  saved = cache serve of a script-stripped snapshot; incurred = cache serve of a snapshot with
  scripts, or any origin serve; unknown = no page record, or a page that predates the upgrade. One
  in-memory counter bump; ~30 combo rows/period/node.

Console (prerender-console-v0.14.0) — originLoad() becomes
net = 1 − (proxied + renders + Σk_renders + probes + sitemaps + Σk_incurred) ÷ (arrived + Σk_saved + Σk_incurred),
the exposure tile becomes "script calls: saved X · incurred Y", the unknown count is printed (it decays
over one render cycle after the plugin deploys), and unspecified is shown as the CDN-default caveat.

Order

Every stage tolerates the others being absent (the console falls back to v0.12.0's exposure count; the
plugin ignores fields an older fleet does not post). Browser first so k starts landing on pages;
plugin; console. Plugin + console land in one PR here (the console's catalog guard scans the plugin's
emit sites, so a plugin-only merge would fail the console suite), released as two tags.

Not resource-intensive

Measurement rides on hooks that already run per response; storage is two small nullable fields per page;
the serve path adds one counter bump for rendering-crawler serves. The cost is coordination and the
one-render-cycle warm-up.