a C ABI, so this can be embedded rather than spawned

Every desktop shell that reads typikon today runs the binary and parses
its JSON. That costs about 1.5ms and is the right answer for a calendar
that changes once a day, but it is the wrong shape for anything with an
FFI and no business spawning a process on every redraw.

Dates cross as Julian Day Numbers, so no struct layout is part of the
contract. A day is opaque and owns its strings, built once when it is
reckoned: an accessor returns the same borrowed pointer every time, so a
statusline polling one in a loop allocates nothing. The accessors carry
every field of the JSON, so nothing on the other side needs a parser.

The Python bindings and this were resolving a jurisdiction by two copies
of the same logic, and the copy here leaked one per call. Both now share
jurisdictions::resolve_full, which interns the combinations instead.

The header is a promise made in a second file, and a second file drifts:
a test reads both and fails if either has a symbol the other does not,
and does the same for the LuaJIT cdef, where a stale declaration is a
segfault rather than a compile error.

examples/lua/typikon.lua

@@ -0,0 +1,168 @@
+-- typikon, through LuaJIT's FFI.
+--
+-- Drop this beside your config and `local typikon = require("typikon")`. It
+-- needs libtypikon.so on the loader's path, or TYPIKON_LIB set to it:
+--
+--     cargo build --release --features capi
+--     TYPIKON_LIB=target/release/libtypikon.so luajit examples/lua/statusline.lua
+--
+-- Written for AwesomeWM, WezTerm and Neovim, which all embed LuaJIT and none
+-- of which should be spawning a process on every redraw.
+
+local ffi = require("ffi")
+
+ffi.cdef [[
+const char *typikon_version(void);
+uint32_t typikon_abi_version(void);
+const char *typikon_last_error(void);
+
+int32_t typikon_jdn_from_ymd(int32_t year, uint32_t month, uint32_t day);
+int typikon_ymd_from_jdn(int32_t jdn, int32_t *year, uint32_t *month, uint32_t *day);
+int32_t typikon_today(void);
+
+typedef struct typikon_day typikon_day;
+typikon_day *typikon_describe(int32_t jdn, const char *jurisdiction);
+typikon_day *typikon_describe_full(int32_t jdn, const char *jurisdiction,
+                                   const char *tradition, int discipline);
+void typikon_day_free(typikon_day *day);
+
+const char *typikon_day_json(const typikon_day *day);
+const char *typikon_day_iso(const typikon_day *day);
+const char *typikon_day_weekday(const typikon_day *day);
+const char *typikon_day_jurisdiction(const typikon_day *day);
+const char *typikon_day_calendar(const typikon_day *day);
+const char *typikon_day_tradition(const typikon_day *day);
+const char *typikon_day_church_date(const typikon_day *day);
+const char *typikon_day_title(const typikon_day *day);
+const char *typikon_day_fast(const typikon_day *day);
+const char *typikon_day_fast_label(const typikon_day *day);
+const char *typikon_day_emoji(const typikon_day *day);
+const char *typikon_day_reason(const typikon_day *day);
+const char *typikon_day_season(const typikon_day *day);
+const char *typikon_day_cycle(const typikon_day *day);
+const char *typikon_day_feast(const typikon_day *day);
+const char *typikon_day_rank(const typikon_day *day);
+const char *typikon_day_sign(const typikon_day *day);
+const char *typikon_day_obligation(const typikon_day *day);
+const char *typikon_day_binds(const typikon_day *day);
+uint32_t typikon_day_commemoration_count(const typikon_day *day);
+const char *typikon_day_commemoration(const typikon_day *day, uint32_t i);
+int typikon_day_is_obligatory(const typikon_day *day);
+int32_t typikon_day_days_to_pascha(const typikon_day *day);
+int32_t typikon_day_pascha(const typikon_day *day);
+int32_t typikon_day_julian_offset(const typikon_day *day);
+
+int32_t typikon_pascha(int32_t year, const char *jurisdiction);
+uint32_t typikon_jurisdiction_count(void);
+const char *typikon_jurisdiction_key(uint32_t i);
+const char *typikon_jurisdiction_name(uint32_t i);
+const char *typikon_jurisdiction_note(uint32_t i);
+]]
+
+local C = ffi.load(os.getenv("TYPIKON_LIB") or "typikon")
+
+local M = { ABI = 1 }
+
+-- A string field, or nil where the library returned NULL. Every accessor may,
+-- and NULL means "the day has no such thing" rather than an error.
+local function str(p)
+  if p == nil then return nil end
+  return ffi.string(p)
+end
+
+local function err()
+  return str(C.typikon_last_error()) or "unknown error"
+end
+
+-- The fields worth reading without a JSON parser. Order matters only for the
+-- table's own readability.
+local FIELDS = {
+  "iso", "weekday", "jurisdiction", "calendar", "tradition", "church_date",
+  "title", "fast", "fast_label", "emoji", "reason",
+  "season", "cycle", "feast", "rank", "sign", "obligation", "binds",
+}
+
+-- Copy the day into a plain Lua table and free it. The C day owns its strings
+-- and they die with it, so nothing here may hold on to a pointer.
+local function harvest(d)
+  local t = {}
+  for _, f in ipairs(FIELDS) do
+    t[f] = str(C["typikon_day_" .. f](d))
+  end
+  t.is_obligatory = C.typikon_day_is_obligatory(d) ~= 0
+  t.days_to_pascha = C.typikon_day_days_to_pascha(d)
+  t.pascha_jdn = C.typikon_day_pascha(d)
+  t.julian_offset = C.typikon_day_julian_offset(d)
+  t.commemorations = {}
+  for i = 0, C.typikon_day_commemoration_count(d) - 1 do
+    t.commemorations[i + 1] = str(C.typikon_day_commemoration(d, i))
+  end
+  t.json = str(C.typikon_day_json(d))
+  C.typikon_day_free(d)
+  return t
+end
+
+--- One day, as a table. `opts` may carry `jurisdiction`, `tradition` and
+--- `typikon` (true to show the day at the Typikon's full discipline, false for
+--- only what particular law requires, nil for whatever was remembered).
+--- Returns nil and a message on error.
+function M.describe(jdn, opts)
+  opts = opts or {}
+  local discipline = -1
+  if opts.typikon == true then discipline = 1
+  elseif opts.typikon == false then discipline = 0 end
+  local d = C.typikon_describe_full(jdn, opts.jurisdiction, opts.tradition, discipline)
+  if d == nil then return nil, err() end
+  return harvest(d)
+end
+
+--- Today, by local time.
+function M.today(opts)
+  return M.describe(C.typikon_today(), opts)
+end
+
+--- The Julian Day Number for a civil date, or nil and a message.
+function M.jdn(year, month, day)
+  local n = C.typikon_jdn_from_ymd(year, month, day)
+  if n == 0 then return nil, err() end
+  return n
+end
+
+--- The civil year, month and day for a Julian Day Number.
+function M.ymd(jdn)
+  local y = ffi.new("int32_t[1]")
+  local m = ffi.new("uint32_t[1]")
+  local d = ffi.new("uint32_t[1]")
+  if C.typikon_ymd_from_jdn(jdn, y, m, d) ~= 0 then return nil, err() end
+  return y[0], tonumber(m[0]), tonumber(d[0])
+end
+
+--- Pascha in a civil year, as a Julian Day Number.
+function M.pascha(year, jurisdiction)
+  local n = C.typikon_pascha(year, jurisdiction)
+  if n == 0 then return nil, err() end
+  return n
+end
+
+--- Every jurisdiction the library knows: { key, name, note }.
+function M.jurisdictions()
+  local out = {}
+  for i = 0, C.typikon_jurisdiction_count() - 1 do
+    out[i + 1] = {
+      key = str(C.typikon_jurisdiction_key(i)),
+      name = str(C.typikon_jurisdiction_name(i)),
+      note = str(C.typikon_jurisdiction_note(i)),
+    }
+  end
+  return out
+end
+
+M.version = str(C.typikon_version())
+
+-- A library newer in a way that changed meaning, not one merely newer.
+if C.typikon_abi_version() ~= M.ABI then
+  error(("typikon ABI %d, this binding wants %d")
+    :format(tonumber(C.typikon_abi_version()), M.ABI))
+end
+
+return M