feat(ru): drop abbreviations from the Russian noun list

The list leaked abbreviations two ways: an OpenCorpora noun tagged Abbr without
Fixd (the declinable acronym ОГВЗ) slipped past the Abbr+Fixd seed filter, and the
РАН «(сокр.)» words (вуз, нэп, колхоз, гост, …) were kept on purpose by the old
policy.

Drop every abbreviation instead. A final pass in ru_stage2.py removes a word when
the orthographic note marks it «(сокр.)» (is_abbrev_note) or when every OpenCorpora
noun reading carries Abbr (oc_abbr_only — also the declinable огвз/заз the seed
filter missed). Homographs with a plain-noun reading survive (ваза «ВАЗ», рис).
Lexicalised words are rescued via manual_confirm.txt (колхоз, совхоз, рация,
спецназ, эсминец, дзот, дот, прораб, токамак, тэн, … and под — a real noun OC knows
only as ПОД); the bound prefix гос is vetoed in manual_reject.txt.

108 words removed from ru_scrabble / ru_erudit; en_sowpods unchanged. tools/README
documents the policy and that libmorph is required for a faithful rebuild.

tools/README.md

@@ -61,7 +61,11 @@ g++ -std=c++17 -O2 tools/libmorph_check.cpp -lmorphrus -lmoonycode -o tools/libm
 ```
 
 If `tools/libmorph_check` is absent, Stage 2 still runs — it simply drops libmorph from
-the stack and reports `libmorph_helper=MISSING`.
+the stack and reports `libmorph_helper=MISSING`. **Build it before a release rebuild:** the
+committed word lists were produced *with* libmorph, and dropping it shifts the result by a few
+hundred words (libmorph-only nouns disappear, and adjectives/eponyms libmorph would veto —
+неперов, венерин — leak in through the orthographic-note path). A rebuild meant to refresh the
+committed lists is only faithful with the helper present.
 
 ## How to run
 
@@ -160,34 +164,38 @@ passed and lemmas returned in UTF-8.
 
 ## Abbreviations, proper nouns & function words
 
-OpenCorpora lists indeclinable abbreviations as common nouns (`Abbr`+`Fixd`), so seeding the
-result with its whole noun lexicon leaked non-words like **ндс, ст, ср, кпд, чп** into
-`scrabble.txt`. They are filtered as follows (see `oc_noun_lemmas`, `oc_abbr_fixd_only`,
-`oc_function_word` and the Stage-2 loop in `ru_stage2.py`):
+**Abbreviations are not Scrabble words and are dropped** (вуз, нэп, гост, жэк, спид, огвз,
+обком, наркомат, роно, тв, …). A final pass in `build` removes every word matched by either
+signal, then re-admits the maintainer's rescues — see `is_abbrev_note`, `oc_abbr_only` and the
+abbreviation subtraction in `ru_stage2.py`:
 
-- An `Abbr`+`Fixd` noun is **never** admitted on OpenCorpora's word alone; the orthographic
-  dictionary decides via its own note (`classify`):
-  - attested as a **lowercase** РАН headword whose note is a noun ⇒ **kept** — the
-    lexicalised abbreviations that are real words: **сельпо, под, ска, роно, врио, тв, фио,
-    суперэвм** (present in `all.txt`, `classify == noun`);
-  - everything else ⇒ **dropped**: bare letter-abbreviations not in `all.txt` (ндс, кпд, чп,
-    гибдд, днк, …) and lowercase РАН headwords whose note is *not* a noun (гор, мин, про —
-    «приставка»; об, по, со — «предлог»; сто — числительное).
-- A word OpenCorpora also reads as a **function word** (`CONJ`/`PRCL`) is dropped even if the
-  note looks noun-like — **зато** (союз), and any like it.
-- A word whose OpenCorpora common-noun reading is an **inflected form** (its lemma is a
-  different word) is not a headword and stays out — **ан** (род. мн. от «ана»; the proper
-  noun «Ан», самолёт Антонова, is capitalised and never reaches `all.txt`).
+- **The orthographic note marks it `(сокр.)`** (`is_abbrev_note`): the РАН dictionary's own
+  abbreviation mark, covering both the bare initialisms (вуз, нэп, гост, бтр, кпд) and the
+  Soviet-era compounds (обком, райком, наркомат, госплан, …). This is the authoritative signal.
+- **OpenCorpora reads it only as an abbreviation** (`oc_abbr_only`): *every* common-noun
+  reading carries the `Abbr` grammeme. This catches acronyms РАН does not head at all — the
+  declinable огвз (a full `Abbr` case paradigm, no `Fixd`, which the `Abbr`+`Fixd` seed filter
+  misses), заз, and the indeclinable роно, тв, фио, суперэвм. A homograph that *also* has a
+  plain-noun reading is **not** abbreviation-only and survives — ваза «ВАЗ», рис «РИС».
 
-The discriminator is **not** the `(сокр.)` mark — сельпо carries it just like ндс:
+Still dropped by the seed/loop before this pass: a `CONJ`/`PRCL` **function word** even with a
+noun-like note (**зато**), and an OpenCorpora reading that is an **inflected form** of another
+lemma (**ан** = род. мн. от «ана»; the proper noun «Ан» is capitalised and never reaches
+`all.txt`). Proper nouns (`Name`/`Surn`/`Geox`/`Orgn`/…) are excluded from the seed outright.
 
-| word | in `all.txt` (lowercase РАН headword) | `classify` (РАН note) | OC `CONJ`/`PRCL` | verdict |
-|------|:---:|:---:|:---:|---------|
-| сельпо, под, ска, роно, врио, тв, фио, суперэвм | yes | noun | no | **keep** |
-| ндс, кпд, чп, гибдд, днк, … | no | — | no | drop (seed) |
-| гор, мин, про, об, по, со, сто, прим | yes | not noun | — | drop (note) |
-| зато | yes | — | **yes** | drop (function word) |
-| ан | yes | noun | yes | drop (OC lemma ≠ word) |
+**Maintainer overrides decide the borderline cases** (this is policy, not a heuristic):
+
+- `manual_confirm.txt` **rescues** lexicalised words that carry `(сокр.)` or that OpenCorpora
+  only knows as an abbreviation but which read as ordinary nouns — **колхоз, совхоз, рация,
+  спецназ, эсминец, линкор, дзот, дот, прораб, токамак, тэн, …**, and **под** (a real noun,
+  «под печи», that OpenCorpora knows only as the abbreviation ПОД). Edit this list to widen or
+  narrow which abbreviations stay.
+- `manual_reject.txt` **vetoes** the few non-noun leftovers that no signal catches — e.g. the
+  bound prefix **гос** («гос… — первая часть сложных слов», not a standalone word).
+
+> History note: an earlier policy *kept* lexicalised abbreviations (сельпо, вуз, нэп, …) and
+> used `(сокр.)` only as a non-discriminator. The current policy drops all abbreviations and
+> rescues a curated subset, so the dictionary holds words, not acronyms.
 
 ### Auditing a word and repeating this analysis