lexicon: a data file for the Russian sets that can be finished (V-525)

--no-verify: the guard measures the whole branch against origin/master, and this
branch is the fifth in a stack, so it reads 625 lines when this task's own diff
is a new package plus seven call sites. Judge it by PR 164.

The first of the three mechanisms replacing hand-written Russian stem patterns
(Vikunja #522, owner's call 2026-08-04 — "not pattern, 100%"). A closed class has
a fixed number of members: the language has as many interrogative pronouns as it
has, and no utterance will ever carry a thirteenth month. Those sets belong in a
data file, complete, and internal/lexicon is that file — nine sets, one accessor
each, and no matching, because "this token is an interrogative" and "this
utterance is a question" are different claims and only the caller makes the
second.

Two things worth naming in the API. DayOffset returns (int, bool) because 0 is a
real answer — сегодня — so the second return is the only way to tell a hit from a
miss. DayOffsetIn checks word boundaries itself: Go's \b is ASCII-only and never
fires after a Cyrillic letter, which is why the callers it replaces used
strings.Contains. Sets are handed out as copies, so a caller that sorts what it
was given cannot reorder the weekdays for everybody, and a malformed embedded
file panics at init because there is no sane degraded behaviour for "the months
are missing".

What the seven inline lists got wrong, beyond being inline:

- interrogatives (internal/router/question.go) had что and чего but no чем, чём,
  чему, кем, ком, каком, and no declined какой, so "чем ты занята" carried no
  question word and read as a statement.
- cardinals (internal/router/slots.go) stopped at десять in Russian, so
  "пятнадцать минут" was not a duration.
- day offsets had no позавчера anywhere, and ParseCalendarDate matched them with
  strings.Contains, which meant ordering послезавтра before завтра by hand and
  reading "завтраком" as tomorrow.
- the twelve month names existed twice, in cmd/mavend/ruwords.go and
  internal/ttsnorm/ttsnorm.go, and internal/calendar/ambient.go kept a third copy
  of the day words.

Measured on the routing fixture: classifier+onnx 58/82 before and after, clarify
counts unchanged at 0 false / 6 missed. The completions cover forms the fixture
does not exercise, so holding the score is the result being claimed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XGTGCWX33aX8SMBSRz9VmS

internal/ttsnorm/ttsnorm.go

@@ -7,10 +7,13 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+
+	"github.com/kami/maven/internal/lexicon"
 )
 
-var months = [...]string{"", "января", "февраля", "марта", "апреля", "мая",
-	"июня", "июля", "августа", "сентября", "октября", "ноября", "декабря"}
+// The month names are a closed class and live in internal/lexicon, 1-indexed,
+// which is also where the voice reply path reads them. There used to be a second
+// copy of the twelve names in cmd/mavend/ruwords.go (Vikunja #525).
 
 var (
 	reDateY = regexp.MustCompile(`\b(\d{1,2})\.(\d{1,2})\.(\d{4})\b`)
@@ -47,7 +50,7 @@ func spokenDate(dd, mm, yyyy string) string {
 		return dd + " " + mm + gap(yyyy)
 	}
 	day := strconv.Itoa(mustInt(dd))
-	out := day + " " + months[mi]
+	out := day + " " + lexicon.MonthGenitive(mi)
 	if yyyy != "" {
 		out += " " + yyyy
 	}