A cell should report its role in a merge

Cluster: core-model

Scenario

Code that walks a worksheet's cells frequently needs to know whether a given cell participates in a merged range and, if so, whether it is the master (top-left, value-bearing) cell or a child (covered) cell. Today that knowledge lives only in the worksheet's merge map; a caller must reach into that map and re-derive the relationship by hand for every cell, which is the exact workaround people write when handling merged ranges, for example when deciding whether to render or skip a cell. The cell already knows enough to answer this; it just does not expose it.

Spec note, not a corpus case: this is a small model addition (two derived accessors), a design decision about what the cell exposes rather than a malformed-output bug. It becomes a corpus case once the accessors exist and their round-trip is asserted.

Desired behavior

• A cell exposes its merge role directly: whether it is part of a merge at all, whether it is the master, and whether it is a child, derived from the worksheet's merge map rather than stored redundantly on the cell.
• The three notions are consistent by construction: a cell in no merge is neither master nor child; every merged range has exactly one master; every other cell of the range is a child.
• A child can reach its master, and the master its range, so a consumer can fetch the display value and style once from the master while iterating children. This complements merged-child-cell-text-mirrors-master, which locks that a child's text already mirrors the master.
• The accessors are cheap, side-effect-free reads, safe to call on every cell of a large sheet.

Open questions

• Naming and shape: a boolean pair (isMerged plus isMergeMaster/isMergeChild) or a single role enum (none | master | child). The enum is harder to misuse, since it has no illegal both-true state.
• Whether the master/range back-reference is eager or lazily resolved against the merge map, and how it behaves if the merge is mutated mid-iteration.
• Whether these live on the cell only, or also as worksheet-level queries (worksheet.mergeOf(addr)).

Related: merged-child-cell-text-mirrors-master, merged-cell-value-and-style-semantics.