Record the Friday Facts blog as a reference source

README.md

@@ -14,6 +14,62 @@ none of the analysis. A probe compares the game against a consumer's own
 reimplementation, so that half has to run in the consumer's language. The
 interface is therefore JSON in and JSON out, not a probe framework.
 
+## Sources
+
+Two kinds, and the difference decides how each may be used.
+
+**Authoritative sources** say what the game accepts. They all ship with the
+install, so a capture reads them offline and gets the same bytes every time.
+These are the only things a capture may depend on.
+
+| Source | Answers |
+| --- | --- |
+| `factorio --dump-data` | Every prototype: names, collision boxes, pipe connections, pole supply and wire reach, beacon stats |
+| `data/*/migrations/*.json` | Every rename, as a table. A complete list, not a guess |
+| `doc-html/runtime-api.json` | The shape of the `defines.*` tables. It publishes no values, so read those with a probe instead |
+
+**Reference sources** say why something changed. They are for a human reading
+after a game update, to work out which captured value now needs review. Nothing
+automated reads them, and nothing automated should.
+
+| Source | Answers |
+| --- | --- |
+| `data/changelog.txt` | Behaviour changes per patch, terse, ships with the install |
+| <https://factorio.com/blog/> | Wube's own Friday Facts posts: the reasoning behind a change, usually weeks before it ships |
+
+### On the Friday Facts blog
+
+It is official, written by the developers, so it outranks forum, wiki and blog
+advice from anyone else. It is still not authoritative in the sense above: it
+describes intent, and the shipped game can differ. Check any claim from it
+against a capture before acting on it.
+
+A worked example. Factorio 2.1 changed the pumpjack's `output_fluid_box`
+pipe connections from two distinct corners repeated, to four distinct corners,
+one per rotation:
+
+```
+2.0.77  positions = [[1,-1], [1,-1], [-1,1], [-1,1]]
+2.1.14  positions = [[1,-1], [1,1],  [-1,1], [-1,-1]]
+```
+
+The dump shows the change. It does not say why. FFF #442, "Flip, Flow, and
+Fresh Paint", is the post that explains it, and the same release added
+`use_mirroring` and `migrate_horizontal_mirroring` to that prototype. Reading it
+is how you learn the change is about entity mirroring rather than a typo fix.
+
+Practical notes, measured 2026-08-17:
+
+- The feed is Atom, at <https://factorio.com/blog/rss>, served as
+  `application/atom+xml`.
+- It carries the **10 most recent posts only**. It is a what-changed-lately
+  feed, not an archive, so it cannot answer "what shipped in version X" on its
+  own.
+- Each entry embeds the full post body, so recent posts need no second fetch.
+- Older posts live at `https://factorio.com/blog/post/fff-<number>`.
+- It is the only source here that needs the network. That is why it can never
+  gate a capture: captures must stay reproducible offline and byte for byte.
+
 ## Scope
 
 Behavioural reverse engineering for interoperability - understanding what the