feat(render): CSV and JSON emitters, and the published contract

Both consume the VIEW, not the kernel, so every emitter and every
template describe exactly the same fields -- there is one vocabulary,
not five.

CSV is RFC 4180: a field with a comma is quoted. That is live on real
data, not hypothetical -- 'St. Joseph, Spouse of the Bl. Virgin Mary'
would otherwise split into two columns.

JSON is hand-rolled because the dependency list is frozen and escaping
is the only subtlety. Control characters below 0x20 are \u-escaped per
RFC 8259 section 7.

There are no numbers in the view, deliberately: a consumer never has to
guess whether week is 2 or "2".

schema/day-v1.json pins the shape. Once a phone subscribes or a site
fetches this, it is a promise to strangers -- adding a field is minor,
renaming one means /v2/.

4 files changed, 110 insertions, 0 deletions

diff --git a/lib/render/emit_csv.ml b/lib/render/emit_csv.ml
new file mode 100644
index 0000000..b478265
--- /dev/null
+++ b/lib/render/emit_csv.ml
@@ -0,0 +1,44 @@
+(* emit_csv.ml *)
+module T = Template
+
+let escape_field s =
+  let needs =
+    String.exists (fun c -> c = ',' || c = '"' || c = '\n' || c = '\r') s
+  in
+  if not needs then s
+  else begin
+    let b = Buffer.create (String.length s + 8) in
+    Buffer.add_char b '"';
+    String.iter (fun c -> if c = '"' then Buffer.add_string b "\"\"" else Buffer.add_char b c) s;
+    Buffer.add_char b '"';
+    Buffer.contents b
+  end
+
+let get v k = match v with T.Obj kvs -> List.assoc_opt k kvs | _ -> None
+let s v k = match get v k with Some (T.Str x) -> x | _ -> ""
+let nested v a b = match get v a with Some inner -> s inner b | None -> ""
+
+let columns =
+  [ "date"; "rite"; "season"; "week"; "slug"; "rank"; "colour"; "subject";
+    "name_la"; "name_en"; "first"; "gospel"; "comms" ]
+
+let row ~rite d =
+  let comms =
+    match get d "comms" with
+    | Some (T.List l) -> String.concat " " (List.map (fun c -> s c "slug") l)
+    | _ -> ""
+  in
+  String.concat ","
+    (List.map escape_field
+       [ s d "iso"; rite; s d "season"; s d "week"; s d "slug"; s d "rank";
+         s d "colour"; s d "subject"; nested d "name" "la"; nested d "name" "en";
+         s d "first"; s d "gospel"; comms ])
+
+let year v =
+  let rite = s v "rite" in
+  let days = match get v "days" with Some (T.List l) -> l | _ -> [] in
+  let b = Buffer.create (64 * 400) in
+  Buffer.add_string b (String.concat "," columns);
+  Buffer.add_char b '\n';
+  List.iter (fun d -> Buffer.add_string b (row ~rite d); Buffer.add_char b '\n') days;
+  Buffer.contents b
diff --git a/lib/render/emit_csv.mli b/lib/render/emit_csv.mli
new file mode 100644
index 0000000..262f660
--- /dev/null
+++ b/lib/render/emit_csv.mli
@@ -0,0 +1,9 @@
+(* emit_csv.mli *)
+(** One row per day, RFC 4180. Consumes the VIEW, not the kernel, so every
+    emitter describes exactly the same fields as every template. *)
+
+(** RFC 4180 section 2: quote a field containing a comma, a quote or a newline;
+    double an embedded quote. Exposed for testing. *)
+val escape_field : string -> string
+
+val year : Template.value -> string
diff --git a/lib/render/emit_json.ml b/lib/render/emit_json.ml
new file mode 100644
index 0000000..272e4ca
--- /dev/null
+++ b/lib/render/emit_json.ml
@@ -0,0 +1,47 @@
+(* emit_json.ml *)
+module T = Template
+
+let escape_string s =
+  let b = Buffer.create (String.length s + 8) in
+  Buffer.add_char b '"';
+  String.iter
+    (fun c ->
+      match c with
+      | '"' -> Buffer.add_string b "\\\""
+      | '\\' -> Buffer.add_string b "\\\\"
+      | '\n' -> Buffer.add_string b "\\n"
+      | '\r' -> Buffer.add_string b "\\r"
+      | '\t' -> Buffer.add_string b "\\t"
+      | c when Char.code c < 0x20 -> Buffer.add_string b (Printf.sprintf "\\u%04x" (Char.code c))
+      | c -> Buffer.add_char b c)
+    s;
+  Buffer.add_char b '"';
+  Buffer.contents b
+
+(* Emit the view tree directly. Bools stay bools; everything else is a string,
+   an array or an object -- there are no numbers in the view, deliberately, so a
+   consumer never has to guess whether "week" is 2 or "2". *)
+let rec write b (v : T.value) =
+  match v with
+  | T.Str s -> Buffer.add_string b (escape_string s)
+  | T.Bool x -> Buffer.add_string b (if x then "true" else "false")
+  | T.List l ->
+      Buffer.add_char b '[';
+      List.iteri (fun i x -> if i > 0 then Buffer.add_char b ','; write b x) l;
+      Buffer.add_char b ']'
+  | T.Obj kvs ->
+      Buffer.add_char b '{';
+      List.iteri
+        (fun i (k, x) ->
+          if i > 0 then Buffer.add_char b ',';
+          Buffer.add_string b (escape_string k);
+          Buffer.add_char b ':';
+          write b x)
+        kvs;
+      Buffer.add_char b '}'
+
+let year v =
+  let b = Buffer.create (64 * 1024) in
+  write b v;
+  Buffer.add_char b '\n';
+  Buffer.contents b
diff --git a/lib/render/emit_json.mli b/lib/render/emit_json.mli
new file mode 100644
index 0000000..91c8f48
--- /dev/null
+++ b/lib/render/emit_json.mli
@@ -0,0 +1,10 @@
+(* emit_json.mli *)
+(** JSON writer. Hand-rolled: the dependency list is frozen (spec section 1),
+    and writing JSON is a page of code where escaping is the only subtlety.
+    Output shape is pinned by schema/day-v1.json, the published contract. *)
+
+(** RFC 8259 section 7, including \u-escaping of control characters below 0x20.
+    Returns the value WITH its surrounding quotes. Exposed for testing. *)
+val escape_string : string -> string
+
+val year : Template.value -> string