docs: add Poll Lane and Sighting to the shared vocabulary

Latest Chapter no longer claims to be discovered without the reader
present: a Sighting establishes it between Polls. Specs for the work
these terms describe are issues #101, #102 and #103.

CONTEXT.md

@@ -56,12 +56,12 @@ is real activity, so only Progress reorders the list.
 _Avoid_: position, bookmark (the noun is taken), last read
 
 **Latest Chapter**:
-The highest-numbered chapter a Site has published for a Series, discovered without the
+The highest-numbered chapter a Site has published for a Series. The number is what
-reader present. The number is what ranks it, never a date and never the Site's own
+ranks it, never a date and never the Site's own "newest chapter" banner — where a Site
-"newest chapter" banner — where a Site disagrees with itself, its list of chapters is
+disagrees with itself, its list of chapters is the record and its summary of that list
-the record and its summary of that list is not. Distinct from Progress in every way
+is not. Established by a Poll and, between Polls, by a Sighting. Distinct from Progress
-that matters: it is a fact about the Site, not about the reader, and it must never
+in every way that matters: it is a fact about the Site, not about the reader, and it
-reorder the list.
+must never reorder the list.
 _Avoid_: newest, current chapter, update
 
 **Poll**:
@@ -70,6 +70,20 @@ Reader present. Performed once per Series no matter how many Readers bookmarked
 a Poll is work done on behalf of the Series, never on behalf of a Reader.
 _Avoid_: scrape, refresh, check, sync
 
+**Poll Lane**:
+One Site's own stream of Polls, carrying the pace at which that Site is willing to be
+asked. Every Site has exactly one and no Lane can slow, block or borrow from another's;
+a Reader never has one and never influences one.
+_Avoid_: worker, queue, scheduler, batch, wave
+
+**Sighting**:
+What a Reader's browser happened to see of a Series's Latest Chapter while that Reader
+was on the page. It reports the same fact as a Poll but carries none of its authority:
+a Poll always overrules it, and only a Sighting on a Series no Reader else holds may
+defer one. A Sighting a later Poll contradicts downwards is a false Sighting, and
+enough of those cost the Reader the right to defer at all.
+_Avoid_: client report, user poll, observation, claim
+
 **Acquisition**:
 The single read of a Series page made the moment the Series first exists, giving it
 both its Latest Chapter and its Cover without waiting out the Poll queue. Distinct