v0.7.0

This release comes out of a review of the whole codebase. Several changes reject input that v0.6.0
accepted, change exit codes, or change what a render draws: read the breaking changes before you
upgrade a script that calls diago.

Breaking changes:

• Every invalid spec exits 1 in every verb (render, diff, check, import), with a structured error
  naming the field: input that is not JSON, empty input (exit 2 before), a missing or unknown type,
  a field of the wrong JSON type, or a validation error.
• Ids are stricter. A node, group, class or package id may not contain -> or end in @ and
  digits (db@2), which read as derived edge ids and layout slots, and no id may hold a control
  character, U+FFFE or U+FFFF.
• direction: UNDEFINED is rejected (exit 1, field direction): write AUTO or leave the
  direction out. This holds for a -previous spec and for an old version read from git too.
• A spec's top-level format and store keys are unknown fields: check reports them as
  unknown-field, render and diff warn, and check -strict and scripts/diago-render --strict
  fail on them. Delete the keys, or add "ignore": ["unknown-field"] to the spec.
• Sequence fragments: a fragment's sections run in order without overlapping, and two fragments
  either nest or stay apart; ranges that cross are a validation error. A section's start and
  end keys match in any case, like every other key, and a null bound is an error ("must be
  given") instead of reading as 0.
• Themes: a theme name is a plain file name, and a name with a path is rejected (exit 1 on the
  spec's theme, exit 2 for -theme). Text art draws no theme, but a theme it is given must still
  load, and check validates the theme too.
• render and diff stop after 10 seconds, exit 2, with a message naming the limit. The limit
  starts once the spec and the -previous file are read.
• A failed write to stdout (a full disk, a closed pipe) exits 2 with error: write output
  instead of exit 0 with a truncated file.
• diago diff -format drawio or -format excalidraw is a usage error (exit 2), not a
  validation error.
• A Windows drive path (C:\specs\arch.json, c:/specs/arch.json) is a path on every system,
  never REV:PATH, so a missing one reports the missing file. A one-letter git revision followed by
  an absolute path (v:/abs/arch.json) is read as a file too: use a relative path, or another
  spelling of the revision (v~0:...).

What a render draws:

• Hops (SVG and PNG): every hop bulges up off a horizontal wire and left off a vertical one,
  whichever way the wire runs; a wire running left bulged down, and one running down bulged right.
  Crossings whose hops would overlap share one hop, a half ellipse no higher than a lone hop. The
  sketch theme draws hops too.
• Every element id in a flow or class SVG is unique: an element whose id would repeat another's
  gets -2, -3. A spec without such a collision keeps every id.
• Text art:
  • Sequence frame headers sit on the frame line (╔══ alt [guard] ══╗, ╟╌╌ [guard] ╌╌╢),
    frame sides keep two blank columns from every lifeline, and a frame leaves six blank columns
    before its first lifeline.
  • A message across several actors gets room for its label instead of writing over its wire.
  • A dashed, dotted or thick vertical wire keeps its style where it crosses another wire.
  • An edge label the layout could not place beside a vertical wire is tried in its own row first.
• A newline, tab or other control character in a single-line text field prints as its escape
  (\n) instead of tearing text art or breaking the SVG. A flow node's label still breaks into
  lines at \n.
• A back edge straightens onto its pinned side column past other wires instead of jogging.
• Exports (beta): every id the exporter makes has a kind prefix, so it never collides with a
  spec's id (draw.io's root and layer cells are root and layer); a self-loop binds its shape
  once; and draw.io and Excalidraw files end in a newline.

Diffs and anchoring:

• A sequence diff keeps the new spec's message order.
• An id that changes kind (a node that becomes a group) is named in the change list by each
  side's own label.
• A removed edge whose generated name collides with a spec's own __removed id is named once, by
  its own ends, and flat-edge-ranked names the right before.edges[i].
• edge-without-id (under diff and render -previous) fires on an edge or class relation
  without an id that shares both its ends with another, labelled or not, since only then can an
  edit hand its derived id (from->to#n) to another edge. It no longer fires on a lone labelled
  edge or a lone relation, and its message says why. check does not change.

Errors and messages:

• render -previous checks the spec first. A field error, an unknown -format, or
  -format drawio|excalidraw on a class or sequence diagram is reported as it is without
  -previous (exit 1 for a field, 2 for a format), and a field error beside -theme nope is the
  field's (exit 1). A -previous file that is missing, or a Mermaid one that does not translate,
  is still reported first.
• Text art reports a spec's field error before its theme, as every other format does.
• Group nesting errors name the group's own field (groups[i]) and come in spec order.
• Validation errors and check -json print <, > and & as written.
• A diff's unknown theme reads error: diff: theme "x" not found.
• A custom theme with a wrongly typed key names it as Theme.<key>.
• -h, --h, -help and --help print a usage on stdout with exit 0 wherever they stand:
  alone, after help, and after version, --version or -version.

PNG:

• Works with resvg 0.26 to 0.41. Those versions printed a warning ahead of the image, so every
  PNG they made was corrupt (exit 0). PNG output needs resvg 0.26 or newer.
• A failed resvg run reports resvg's error alone, a run stopped by the 10-second limit reports the
  limit, and DIAGO_RESVG_PATH naming a directory says so.

Mermaid import:

• An @ inside an edge label, %% inside a quoted label, nested generics (List~List~int~~)
  and generics of non-identifiers (Result~[]User~) import correctly, and the import's JSON keeps
  <, > and & as written.

Plugin:

• The SessionStart hook prints valid JSON when the plugin's path holds a tab, a newline or
  another control character.
• The diagramming skill and its cheatsheet state the new exit codes, id rules, theme check and
  fragment rules.