v0.86.0

Documents now render even when incomplete, the canonical card-yaml fence
becomes a bare ~~~, and the way placeholder/illustrative values are
produced is reworked. This release also fixes two markdown→Typst
conversion bugs and stamps a PDF /Producer field.

Breaking changes

• Bare ~~~ is now the canonical card-yaml fence (was ~~~card-yaml)
  (#662). Existing ~~~card-yaml documents still parse, but to_markdown
  re-emits the bare ~~~ form, so a document's canonical bytes change on
  its first re-emit (relevant if you content-hash or byte-compare emitted
  markdown, or store blueprint goldens). A side effect: a column-zero
  ~~~ fence in a prose body is now read as a card-yaml block — use a
  backtick fence or a non-card-yaml info string (e.g. ~~~rust) for a
  literal code block. Full details and corpus-migration steps:
  docs/migrations/0.85-to-0.86.md.
• fill_blueprint() removed from quillmark_core and quillmark,
  along with its re-exports (#657, #665). Callers no longer post-process a
  blueprint string: fillable/illustrative documents come from
  QuillConfig::example(), and the render path fills placeholders itself
  (see below).

Behavioral changes

• Incomplete documents render instead of erroring (#665). An absent
  Must Fill field is no longer a render error. On the render path each
  schema field resolves to its authored value, else its default:, else a
  type-empty zero value — applied to the plate projection only, never
  persisted to the document. Only malformed input stays fatal: a surviving
  <must-fill> sentinel, or a value that won't coerce/validate.
  quill.form(doc) still reports completeness independently of the render
  gate.
• default vs example clarified (#665, #663, #658). default is the
  value most authors want and is interpolated when a field is omitted (an
  authored value always wins); example documents a field's shape only and
  never renders into output. Preview and illustrative fills now draw from a
  field's example: when present, falling back to the leanest type-valid
  value ("", 0, false, [], {}, first enum variant, empty body).

Markdown → Typst fixes (#661)

• Code is now emitted as #raw(...) with a string literal instead of a
  backtick fence. This fixes fenced or inline code whose content contained
  a run of three-or-more backticks, which previously closed the block early
  and rendered as markup.
• Ordered-list start numbers are preserved — a list written 3. / 4. now
  renders starting at 3 instead of restarting at 1.

New API

• QuillConfig::example(), plus example getters on the Python and WASM
  bindings (#665).
• quillmark_core::zero_value — the single source of truth for a field's
  type-minimal value, shared by blueprint emission and the render path
  (#665).
• RenderOptions.producer on the core, WASM, and Python render APIs (#656)
  — overrides the PDF /Info /Producer string, which now defaults to
  Quillmark <version> on every Typst-rendered PDF.

Other fixes

• PDF rendering folds the /Producer stamp and the signature-field
  AcroForm injection into a single incremental-update pass, preserving
  Typst's /Creator (#656).
• usaf_memo: the signature widget is now overlaid at the 4.5in signature
  block (AFH 33-337) instead of the 1in left margin, and no longer consumes
  layout flow that could push the block out of position (#660); empty
  signature fields no longer carry the APPEND_ONLY flag (#654).