v0.4.0

Model-driven reusable-fragment population completes the comprehension vertical,
plus native SmartArt and PowerPoint cell merges. Brand Profiles from 0.1.x-0.3.0
keep working unchanged; an absent comprehension is still the deterministic path.

Added

• The reusable-fragment registries (components / sections) are now
  populated by the model through the fail-closed comprehend boundary - no
  hardcoded catalog. A comprehend proposal may carry a fragments list (each
  {ref, kind: component|section, purpose?, blocks}); merge validates it
  fail-closed (every block parses as a known IID primitive; a nested
  component/section ref must resolve to another fragment proposed in the same
  comprehension; (kind, ref) is unique; cyclic references are rejected) and, on
  a clean pass, DERIVES the entries into profile['components']/['sections'] -
  the registries expand_components already inlines. The registries are rebuilt
  deterministically from the (single-source) comprehension on every clean merge,
  so re-comprehending the same proposal is byte-idempotent and a fragment-less
  comprehension leaves them empty. A fragment is presentation-free IID, so a
  validated proposal resolves through the same brand chokepoint as inline content
  and cannot widen the brand guarantee. The comprehend-input bundle now surfaces
  advisory fragment_candidates (recurring layouts/styles derived cheaply from
  the existing artifact_catalog, possibly empty) as a hint; the model does the
  semantic detection. The shared comprehension prompt gains a fifth question + a
  worked example, byte-identical across the three skills.
• Fragment slots are now substituted: a {{name}} token in a fragment's
  template text is filled from the referencing component/section block's
  slots at expansion time (an unfilled or null token resolves to the empty
  string and is never leaked). Substitution deep-copies, so it never mutates the
  shared profile registry.
• PowerPoint tables now honor colspan/rowspan: a spanning cell (e.g. a banner
  across columns) merges the covered grid cells in the native table, matching the
  docx table writer. Previously pptx rendered a full ungrouped grid while docx
  merged.
• smartart blocks are now authored as a NATIVE, on-brand diagram instead of
  degrading to text - completing native rendering for every block type. On pptx
  the diagram is real autoshapes (a chevron row for a process/flow, a stacked
  rounded-box list otherwise), inheriting the deck theme's accent fill. On docx
  it is a brand-styled table (a process is a single row, one cell per step; a list a
  single column, one row per node). A node's children are preserved (sub-lines in
  the pptx shape, inlined in the docx cell); an empty diagram degrades loudly. The
  diagram is rendered (not editable SmartArt with the layout-engine parts).