feat(document): answer which cells a formula reads

[andiwand]

🤖 Generated with Claude Code

Stacked on #885, which is stacked on #884 — this PR's diff is the third commit.

Step 3.2 of docs/design/spreadsheet-editing.md.

What it does

document.dependents(SheetPosition(0, 0, 0));   // who reads Sheet 1's A1?
document.dependents({a, b, c});                // one walk for a batch of edits
document.unresolved_formulas();                // who might read anything?

internal::SheetDependencies walks every sheet, parses every formula, and resolves each reference to the rectangle of a sheet it reads. A query then walks that reverse map transitively, so a formula reading a formula is named too.

Where it lives, and why

The graph is built the first time it is asked and kept on the document: writing a formula is refused until step 4, so nothing a write does changes it. It is reached through abstract::Document::sheet_dependencies() and held by internal::Document, which every engine derives from — so no engine writes the graph itself.

What each engine does write is one hook: SheetAdapter::sheet_visit_formulas, which hands out the cells the file spells rather than the positions they cover. That distinction is not cosmetic — a repeated ODS row of 1024 columns over 1048576 rows is a handful of nodes and a billion positions. odf walks the runs the parser indexed, ooxml its cell map, and an engine that drops the expression at parse time (.xls, .numbers) visits none.

The honest answer for what it cannot read

A formula that names something no position can be read out of — one that does not parse, a named expression (SUM(Sales)), a reference over several sheets (Sheet1:Sheet3!A1) — is in unresolved_formulas() rather than silently reading nothing. It may read anything, and a caller that has to be right (#888, dropping a stale cached value in an .ods) treats those as dependent on everything.

A reference into another document is neither a dependent nor unresolved: no edit here reaches it.

Public surface

SheetPosition — the sheet by its place among the document's sheets, which is how an edit operation names one, and the cell by TablePosition. Ordered and hashable, like its neighbour.

Not bound yet (python/jni/apple/wasm). The host-facing shape of this is step 4's recalculate(operations) → changed cells; binding the graph query on its own can follow if it's wanted sooner.

Test

14 cases in test/src/sheet_dependencies_test.cpp, from inline .fods and .xlsx fixtures: the chain, ranges, whole columns, cross-sheet by name (without case), the batch, both unresolved kinds, and that every member of a shared group reads its own row.