Add `Pretty` structure, a Wadler-Lindig pretty-printer engine

[julianhyde]

Add Pretty.java to net.hydromatic.morel.util — a self-contained implementation of Wadler's "prettier printer" algorithm (the Wadler-Lindig variant with Leijen's Column/Nesting extensions).

This is Part B of the morelfmt formatter (#281). It has no dependency on Morel's AST and could later be extracted to a standalone library or reimplemented in Morel itself.

Design

Pretty is a utility class containing the Doc algebraic data type (representing a set of possible layouts for a document) and a layout engine that chooses the best layout given a line-width limit.

Internal Doc variants

Variant	Meaning
Empty	The empty document
Text(s, doc)	Literal string s followed by doc
Line(indent, doc)	Newline, then indent spaces, then doc
FlatAlt(primary, flat)	primary when broken; flat when flattened
Concat(a, b)	a followed by b
Nest(indent, doc)	Increase indentation by indent
Union(wide, narrow)	Choose wide if it fits, else narrow
Column(fn)	Access current column (for align)
Nesting(fn)	Access current nesting level

Public combinators

Primitives:
empty, text, line, lineBreak, softLine, softBreak, hardLine

Composition:
beside (concatenation), nest, group (try flattening), align (align to current column), hang, indent, flatten

List combinators:
hsep, vsep, sep, hcat, vcat, cat, fillSep, fillCat, punctuate, encloseSep

Bracketing helpers:
parens, braces, brackets

Rendering:
Pretty.render(int width, Doc doc) → String

Layout algorithm

Best-fit with bounded lookahead via fits. This is the standard approach used in wl-pprint, paiges, and prettyprinter. The Column and Nesting constructors (Leijen's extension) enable align, which is essential for aligning | in match arms and , in tuples/records.

File plan

File	Purpose
src/main/java/net/hydromatic/morel/util/Pretty.java	Pretty class with Doc type, combinators, and render
src/test/java/net/hydromatic/morel/util/PrettyTest.java	Unit tests

Verification

• Test each combinator (group, nest, align, sep, cat,
  encloseSep) with various widths (e.g. render(40, doc) vs
  render(80, doc)).
• Verify flatten invariants.
• mvn test passes.

References

• Wadler, "A prettier printer" (2003)
• Lindig, "Strictly Pretty" (2000)
• Leijen, wl-pprint (Column/Nesting extensions)