feat: surface chart-default and overlay yaml comments under Show docs toggle

[bosiakov]

• Add DefaultComment field on FlatValueEntry, populated by newFlatValues, kept separate from Comment so the chart-side annotation survives RebuildUnifiedEntries rewriting Comment to overlay-side text
• Render DefaultComment as a caption above each row in the left (chart) panel when ShowDocs is on
• Set font size for left and right comment surfaces to 12sp
• Rename Show comments to Show docs across the checkbox label, Go API, layout constants, and the showDocs JSON preference key
• Promote a defaults entry from {} or [] placeholder to a section when the overlay populates that key, so the section comment editor applies and children group under it
• Add IsEmptyContainer helper on FlatValueEntry along with emptyMapValue and emptyListValue constants
• Walk the parent chain in SortByFilePositions when an entry has no recorded position so overlay-added descendants sort beside the section header rather than the alphabetic tail
• Index Comment and DefaultComment in the search cache so queries match documentation prose from either source
• Remove the + chip on extras rows and delete extra_chip.go; cyan tint and the placeholder text still signal extras status
• Change extras placeholder from not in chart glyph form to plain not in chart and render it for both leaf and section extras rows