Reuse parsed value enclosures by default #592

Problem

RemoveEnclosingMiddleware already records whether each parsed value used braces, quotes, or no enclosure. The default writer nevertheless configures AddEnclosingMiddleware(reuse_previous_enclosing=False), so an unchanged parse/write cycle converts quoted and bare numeric values to braces.

That creates unrelated diffs and means editing a field's value also changes its established enclosure style. This is avoidable layout mutation because the parser already has the necessary metadata.

Proposed default

Set reuse_previous_enclosing=True in the default unparse stack:

• parsed quoted values remain quoted;
• parsed braced values remain braced;
• parsed bare integers remain bare;
• explicitly demanded bare macro/expression values remain bare; and
• new programmatic fields without parser metadata continue using braces.

Safety for edited bare values

Reusing no-enclosing metadata requires one guard. A bare integer may later be edited to ordinary text while retaining the original metadata. Emitting that text bare could create invalid BibTeX. When no-enclosing comes only from parser metadata (rather than an explicit field.enclosing demand), reuse it only while the current value is still numeric; otherwise fall back to the configured default enclosure.

Tests should cover unchanged quote/brace/bare values, edits to quoted and braced fields, the edited-bare fallback, and values created without metadata.

Compatibility

This is a formatting-default change, not a semantic transformation. Callers who prefer uniform braces can continue to supply AddEnclosingMiddleware(reuse_previous_enclosing=False, ...) explicitly. Preserving source enclosure choices is the less surprising default for ordinary read/edit/write use.