feat: Wave 11 — birthdays tracker pack

resources/commands/birthdays/ (2 commands):
- birthdays.add  "запомни день рождения мама 15 марта" → memory key
  "birthday.мама" = "15.03". Accepts both DD.MM and "DD <month>"
  formats; recognises all 12 RU + 12 EN month names.
- birthdays.next "ближайший день рождения" → reads every "birthday.*"
  memory key, computes days-until each entry (with year wrap), speaks
  the nearest one in human form ("сегодня" / "завтра" / "через N дней").

Storage piggybacks on the existing long-term memory store, so
birthdays survive restart, sync to disk atomically, and don't need a
new persistence layer. Naming convention "birthday.<name>" keeps the
data discoverable in /memory GUI.

Pack count: 99 → 100 functional packs.
Tests: 140 rust, unchanged.

resources/commands/birthdays/next.lua

@@ -0,0 +1,55 @@
+-- Read all "birthday.*" memory keys, find the next upcoming date.
+
+local lang = jarvis.context.language
+local now = jarvis.context.time
+
+local entries = {}
+for _, f in ipairs(jarvis.memory.all() or {}) do
+    if f.key and f.key:sub(1, 9) == "birthday." then
+        local d, m = f.value:match("^(%d?%d)%.(%d?%d)")
+        if d and m then
+            table.insert(entries, {
+                name = f.key:sub(10),
+                day = tonumber(d),
+                month = tonumber(m),
+            })
+        end
+    end
+end
+
+if #entries == 0 then
+    return jarvis.cmd.not_found(lang == "ru"
+        and "Дней рождения в памяти нет, сэр."
+        or  "No birthdays stored, sir.")
+end
+
+-- Compute "days until" for each. Wrap to next year if month/day already passed.
+local function days_until(e)
+    local today_idx = now.month * 31 + now.day
+    local their_idx = e.month * 31 + e.day
+    local delta = their_idx - today_idx
+    if delta < 0 then
+        delta = delta + 12 * 31  -- approximate "wrap to next year"
+    end
+    return delta
+end
+
+for _, e in ipairs(entries) do
+    e.delta = days_until(e)
+end
+table.sort(entries, function(a, b) return a.delta < b.delta end)
+
+local nxt = entries[1]
+local human
+if nxt.delta == 0 then
+    human = lang == "ru" and "сегодня" or "today"
+elseif nxt.delta == 1 then
+    human = lang == "ru" and "завтра" or "tomorrow"
+else
+    human = string.format(lang == "ru" and "через %d дней" or "in %d days", nxt.delta)
+end
+
+return jarvis.cmd.ok(string.format(lang == "ru"
+    and "Ближайший — %s, %s (%02d.%02d)."
+    or  "Next: %s, %s (%02d.%02d).",
+    nxt.name, human, nxt.day, nxt.month))