Record the lightnovelworld series-identity decision (#77) (#81)

Docs only. No code, no tests, nothing to run. Implementation is specified in #80.

Outcome of a grilling session on 2026-08-11 against #77, backed by live measurement of lightnovelworld over 2026-08-10/11.

## What changed

**`docs/adr/0008-series-identity-is-discovered-not-derived.md`** (new)

A Series identity is discovered from the Site's own links, never derived from an address.
On lightnovelworld the userscript reads the chapter page's `All Chapter` anchor instead of
building a `/novel/<slug>/` address by string manipulation. A Chapter Slug is not an
identity and is not stored. The backend's chapter scan drops its per-Series scoping and
runs against the body truncated before the visitor comment thread.

Evidence in the ADR: 3 of 41 sampled novels serve chapters under a slug that differs from
their series slug, divergence runs in both directions, one novel serves chapters under two
slugs, and neither slug is computable from the other. The pointer was checked on 8 chapter
pages and agreed every time. Three narrower selectors are recorded as rejected, each with
the measurement that killed it.

Three rejected options are recorded with reasons: correcting the stored address only, which
keeps an identity the Site does not guarantee; scoping the scan to a container, which the
probe refuted; and a SQL migration, which is impossible because the database holds no
source for the correct slug.

**`CONTEXT.md`**

- **Series** - identity is the canonical slug the Site publishes, never the title and never a Chapter Slug.
- **Chapter Slug** - new term. A slug a Site builds its chapter addresses from. Not an identity: one Series may have several, and none is computable from another.
- **Latest Chapter** - now the highest-numbered chapter, explicitly not a date and not the Site's own newest-chapter banner. Settles #79.

**`docs/research/lightnovelworld-chapter-vs-series-slug.md`** (new, committed with its corrections)

The 41-novel survey behind the ADR. Two claims are struck through and corrected in place,
with the date and sample size of the probe that refuted each: 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, because that region is writable by
any visitor while the scan takes an unbounded maximum into a Series row shared by every
Reader (ADR-0003).

## Review notes

Nothing here constrains code that exists today - the ADR describes work not yet written.
The part worth disagreeing with, if any of it is wrong, is the fail-closed rule: a missing
truncation marker means skip the Series and log, never scan the whole page.

Related: #77 (the defect), #80 (the spec), #79 (the numbering anomaly, closed by decision),
#71 (the same size cap seen from the cover side).

Reviewed-on: #81
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

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**: