prose: human-readable projection renderer (iter 20a)

New crate ailang-prose with one public fn module_to_prose(&Module)
-> String. Rust-flavour with braces, =>-match-arms, mode keywords
(own/borrow), effects as trailing 'with IO', Cons(1, Nil) ctor
form, /// doc strings.

Lossy projection where the LLM can re-derive: (con T) wrap,
(fn-type ...) wrap, (term-ctor ...) wrap. Load-bearing semantic
detail stays visible (modes, effects, clone, reuse-as, doc strings,
type annotations, tail flag).

CLI: 'ail prose <file.ail.json>' prints the projection.

3 snapshot fixtures + 28 unit tests. 215-line .ail.json reduces
to ~18 lines of legible source.

20b queued: infix arithmetic, paren elision, let-inlining, do
prettify.

Cargo.lock

@@ -9,6 +9,7 @@ dependencies = [
  "ailang-check",
  "ailang-codegen",
  "ailang-core",
+ "ailang-prose",
  "ailang-surface",
  "anyhow",
  "clap",
@@ -48,6 +49,13 @@ dependencies = [
  "thiserror",
 ]
 
+[[package]]
+name = "ailang-prose"
+version = "0.0.1"
+dependencies = [
+ "ailang-core",
+]
+
 [[package]]
 name = "ailang-surface"
 version = "0.0.1"