feat(compiler): list literals, typed compound assignment, file:line diagnostics

- `[a, b, c]` list literals (E_LIST → emit_list); static_type learns
  slice-element, `new T`, list, string and literal kinds
- `x op= y` lowers through the same path as `x = x op y` (emit_bin_vals):
  fixed `*=`/`/=` use the Q16.16 64-bit paths, string `+=` concatenates,
  int→long widens; unary `-` keeps a fixed operand's type (arith_ty)
- one `unescape()` table for "strings", 'chars' and `interpolation`;
  `'\''`, `'\\'`, `'\"'` no longer read as 0; unterminated char literals
  and unexpected characters are errors instead of silently skipped
- every diagnostic is `file:line: error: msg` (g_parse_file / g_err_file,
  Node.file + Node.line set by node()); tok_desc() in expectation errors;
  duplicate `function` names and unknown `phase` names are reported in
  source terms (phase_id used to default unknown phases to Overlay)
- interpolation holes skip braces inside string literals
- hand-IR preludes move from the user `@fn_` prefix to `@lp_` so a user
  `is_ws` / `str_eq` / `path_join` no longer collides at link time
- `@ClearColor(expr)` accepts any constant expression; `Os.pid()` added
  (docs page + inventory); `str_starts()` in support/str
- main.ludic: `else if` flag ladder, char literals, stale script comments
- examples/lang/operators.ludic covers all of the above; os.ludic covers
  Os.pid; docs pages for Os.pid and the Overlay phase; ten changesets
- reseeded: selfhost/ludicc.seed.ll is the new compiler's own fixpoint

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

selfhost/support/buf.ludic

@@ -13,17 +13,17 @@ function buf_new() -> Buf {
 }
 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 }
+  while b.len + extra + 1 > 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
+  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 }
+  while s[i] != 0 { buf_putc(b, s[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 }