Record the lightnovelworld series-identity decision (#77)

A Series identity is discovered from the Site's own links, never derived
from an address. ADR-0008 carries the decision and the evidence behind
it: 3 of 41 sampled novels serve chapters under a slug that differs from
their series slug, one novel serves chapters under two, and neither slug
is computable from the other.

CONTEXT.md gains Chapter Slug as a term, defines it as plural by nature
and never an identity, and pins Series identity to the canonical slug the
Site publishes. Latest Chapter is restated as the highest-numbered
chapter rather than the newest-dated one, settling #79.

The research note is corrected in place where later probes refuted it:
the ul.clstyle container it named is the hidden empty "Latest Reading"
template rather than the chapter list, and its caveat about the comment
region understated the risk, since that region is writable by any
visitor and the scan takes an unbounded maximum into a shared row.

Docs only; no code. Implementation is specified in #80.

CONTEXT.md

@@ -7,17 +7,24 @@ so progress survives across sites and devices.
 ## Language
 
 **Series**:
-One ongoing work — a manga or a novel — as published by a Site. Identified by its
-stable slug on that Site, never by its title. A Series exists once and is shared by
-every Reader who bookmarks it; it owns the facts that are true regardless of who is
-reading — title, cover, Latest Chapter. A Reader cannot change them; they describe the
-Series, not anyone's relationship to it.
+One ongoing work — a manga or a novel — as published by a Site. Identified by the canonical
+slug the Site itself publishes for it, never by its title and never by a Chapter Slug. A
+Series exists once and is shared by every Reader who bookmarks it; it owns the facts that
+are true regardless of who is reading — title, cover, Latest Chapter. A Reader cannot
+change them; they describe the Series, not anyone's relationship to it.
 _Avoid_: manga, title, book, comic
 
 **Site**:
 One third-party source a Series is published on. A Series on two Sites is two Series.
 _Avoid_: source, host, provider, domain
 
+**Chapter Slug**:
+A slug a Site builds its chapter addresses from. Not an identity: one Series may have
+several, any of them may differ from the slug that identifies the Series, and none is
+computable from another. Only the Site's own links say which ones a Series uses, so a
+Chapter Slug is always discovered, never derived.
+_Avoid_: series slug, url slug, permalink, chapter path
+
 **Cover**:
 The image that stands for a Series wherever it is listed. A fact about the Series like
 its title — one Cover per Series, shared by every Reader, never per-Reader. Defined by
@@ -49,9 +56,12 @@ is real activity, so only Progress reorders the list.
 _Avoid_: position, bookmark (the noun is taken), last read
 
 **Latest Chapter**:
-The newest chapter a Site has published for a Series, discovered without the reader
-present. Distinct from Progress in every way that matters: it is a fact about the Site,
-not about the reader, and it must never reorder the list.
+The highest-numbered chapter a Site has published for a Series, discovered without the
+reader present. The number is what ranks it, never a date and never the Site's own
+"newest chapter" banner — where a Site disagrees with itself, its list of chapters is
+the record and its summary of that list is not. Distinct from Progress in every way
+that matters: it is a fact about the Site, not about the reader, and it must never
+reorder the list.
 _Avoid_: newest, current chapter, update
 
 **Poll**: