refactor(selfhost): reorganise into concern-based subdirectories

Split the flat 38-file selfhost/ into concern-based subdirectories:

  frontend/        lex, parse, parse_game, ast
  support/         str, buf, io
  backend/         core IR + expression/statement lowering
  backend/game/    ECS/scene/event/world lowering
  backend/stdlib/  the namespaced Math.*/Text.*/Crypto.*/… intrinsics

and split the three oversized emitters at responsibility boundaries so
no file mixes concerns:

  emit_game.ludic  -> + emit_world.ludic         (reflection world table,
                                                  tick helpers, @main synthesis)
  emit_expr.ludic  -> + emit_call.ludic          (namespaced builtins, call
                                                  lowering, expr dispatch)
  emit_text.ludic  -> + emit_text_prelude.ludic  (emitted string-builder runtime)

FRAGS in tools/x/selfhost.ludic is updated to the new paths with the link
order preserved, and the Python doc/vocabulary tooling is updated to walk
the new layout. Because the build is a plain in-order concatenation and
every split lands on a blank-line boundary, the regenerated seed is
byte-identical: `x reseed` leaves selfhost/ludicc.seed.ll unchanged,
`x bootstrap-cfree` still reaches its fixed point, and both `x test` (56)
and `x selfhost-test` (29, incl. golden renders) stay green.

Closes #29

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/support/buf.ludic

@@ -0,0 +1,29 @@
+# buf.ludic — a growable byte buffer for building IR text. The emitter writes
+# module-level material (types, globals, string constants) into one buffer and
+# function bodies into another, then prints them in order.
+
+property Buf { data: pointer = null, len: int = 0, cap: int = 0 }
+
+function buf_new() -> Buf {
+  let b = new Buf
+  b.cap = 256
+  b.data = bytes(b.cap)
+  b.len = 0
+  return b
+}
+function buf_ensure(b: Buf, extra: int) -> void {
+  if b.len + extra + 1 <= b.cap { return }
+  while b.len + extra + 1 > b.cap { b.cap = b.cap * 2 }
+  b.data = resize(b.data, b.cap)
+}
+function buf_putc(b: Buf, c: int) -> void {
+  buf_ensure(b, 1)
+  b.data[b.len] = c
+  b.len = b.len + 1
+}
+function buf_puts(b: Buf, s: pointer) -> void {
+  var i = 0
+  while s[i] != 0 { buf_putc(b, s[i]); i = i + 1 }
+}
+function buf_puti(b: Buf, n: int) -> void { buf_puts(b, itoa(n)) }
+function buf_str(b: Buf) -> pointer { b.data[b.len] = 0; return b.data }

selfhost/support/io.ludic

@@ -0,0 +1,22 @@
+# io.ludic — reading the input file and writing the output, plus tiny stdio.
+# The compiler reads one .ludic file whole and emits LLVM IR text to stdout.
+
+function read_file(path: string) -> pointer {
+  let f = file_open(path, "rb")
+  if (f == null) { return null }
+  file_seek(f, 0, 2)
+  let n = file_tell(f)
+  file_seek(f, 0, 0)
+  let buf = bytes(n + 1)
+  file_read(f, buf, n)
+  buf[n] = 0
+  file_close(f)
+  return buf
+}
+
+# length of a NUL-terminated buffer
+function cstr_len(s: pointer) -> int {
+  var n = 0
+  while s[n] != 0 { n = n + 1 }
+  return n
+}

selfhost/support/str.ludic

@@ -0,0 +1,33 @@
+# str.ludic — the string handling a compiler lives on: comparison, copying,
+# slicing out substrings, and integer<->text. All over raw NUL-terminated
+# byte buffers reached with peek8/poke8.
+
+# a fresh NUL-terminated copy of src[start .. start+n]
+
+function char_is_digit(c: int) -> bool { return c >= 48 and c <= 57 }
+function char_is_alpha(c: int) -> bool {
+  if c >= 65 and c <= 90 { return true }
+  if c >= 97 and c <= 122 { return true }
+  return c == 95
+}
+function char_is_alnum(c: int) -> bool { return char_is_alpha(c) or char_is_digit(c) }
+
+# integer -> fresh decimal string
+function itoa(v: int) -> pointer {
+  if v == 0 { let z = bytes(2); z[0] = 48; z[1] = 0; return z }
+  var neg = false
+  var x = v
+  if x < 0 { neg = true;  x = 0 - x }
+  let tmp = bytes(16)
+  var n = 0
+  while x > 0 { tmp[n] = 48 + x % 10;  x = x / 10;  n = n + 1 }
+  var total = n
+  if neg { total = total + 1 }
+  let out = bytes(total + 1)
+  var k = 0
+  if neg { out[0] = 45;  k = 1 }
+  var i = 0
+  while i < n { out[k + i] = tmp[n - 1 - i];  i = i + 1 }
+  out[total] = 0
+  return out
+}