Conditional formatting: string-literal quoting and per-cell expression translation

Cluster: conditional-formatting

Scenario

A user wants a fill applied to cells whose value equals a short string (say "T") via a conditional-formatting rule. They reach for a cellIs rule with operator equal and a formulae entry of T, but the resulting workbook applies the fill to the wrong cells, notably all blank cells, because the bare token T is emitted as a defined-name or reference rather than a quoted string literal. Users find that switching to an expression rule with a relative formula like A2="T" works, because the relative address, anchored at the range's top-left, is translated per-cell across the range so each cell is compared to its own value. A separate confusion in the thread is a viewer rendering difference between applications, not a library defect.

Spec note, not a corpus case: the durable value is the CF-rule authoring contract, meaning how a string literal must be quoted and how relative references in an expression rule are translated, rather than a single malformed-output assertion. The corpus already round-trips CF rules (databar-conditional-formatting-roundtrip, conditional-formatting-multi-area-ref-survives, extension-only-conditional-formats-round-trip); this note pins the semantics.

Desired behavior

• cellIs string literals are quoted. For a cellIs rule with equal, and the other comparison operators, an operand intended as a string literal is serialized as an OOXML quoted string ("T"), not a bare token Excel resolves as a defined name or reference. A bare unquoted token currently produces a formula that matches empty and blank cells, a silent correctness bug.
• Expression rules translate relative references per-cell. For an expression rule, formulae with relative cell references such as A2="T" are anchored at the top-left cell of the rule's ref range and translated per-cell across the whole range, matching Excel's shared-formula-style relative translation for conditional formatting. This is what makes a per-row rule evaluate each row against its own cell.
• The API makes the string-literal-against-reference distinction unambiguous so a caller does not have to know OOXML quoting rules to compare against a literal.

Open questions

• Does the library auto-quote a cellIs operand that is a plain string (least-surprise), require the caller to pass a quoted literal explicitly, or offer a typed {literal: 'T'} against {ref: 'A2'} discriminator?
• How is numeric-against-string operand intent disambiguated for cellIs equal, meaning compare-to-number against compare-to-text?
• Per-cell translation: is the anchor always the range's top-left, and how are absolute $ references left untranslated as expected?

Related: databar-conditional-formatting-roundtrip, conditional-formatting-multi-area-ref-survives, extension-only-conditional-formats-round-trip, conditional-format-numfmt-roundtrip.