stack-encrypt: name the operator, not the index, in a query (Eq and Matches) #1182

Background

ADR-0009 (#1177) has the caller name an operator rather than an index. Today a query is cipher.query(value).using(&plan).index(Match(..)) or .equality(): the caller picks the index, and nothing in the types says which operators an index answers.

Problem

• A caller has to know which index answers which operator.
• An operator an index cannot answer is not a compile error.
• The query term is typed as the stored term (MatchTerms for both), so the API has nowhere to say that a query differs from what is stored.

Proposal

Add an operator-typed query to packages/stack-encrypt for the two operators that stack-encrypt's indexes answer today.

• A stack_encrypt::op module with the operator unit types Eq and Matches<C = Insensitive>, the case markers Insensitive and Sensitive, and the traits Operator, QueryTerm, Supports<Op, S> and Answers<Op, S> (ADR-0009 decisions 1 to 4).
• Supports<Eq, S> for Equality, whose query term is the EqualityTerm. Supports<Matches<C>, S> for Match<O> when O's case is C, whose query term is a new MatchQuery<O>, distinct from the stored MatchTerms<O>.
• A case-sensitive Matches against a case-folding match index is a compile error.
• .op(op) on all three plan shapes: a field of a fields plan, a one-value plan with indexes, and a one-value plan with a target. A target answers through Answers<Op, S>, chosen by hand per type. A plan that declares indexes answers through the operator's default index. .op_via(op, index) names a different index, such as a match index with its own configuration.
• Remove QueryUsing::equality(); callers use .op(op::Eq).
• Keep QueryUsing::index(..). It is still the only way to query ORE and OPE, and stack-encrypt: ordering operators Lt, Lte, Gt, Gte and Between, and remove QueryUsing::index #1186 removes it once the ordering operators exist.
• Update ADR-0009's code to the spelling this ships.
• Tests: the Eq query is byte-equal to the stored equality term, a Matches query is contained in the stored filter, and an undeclared index, a wrong plaintext type, an unsupported operator and a case mismatch are each refused, the last two at compile time (tests/ui).

Out of scope

The EQL types implementing Answers and serialising a query term to an eql_v4.query_* domain (decision 8). On this base the v4 types are not Stack Encrypt targets yet; that waits for #1179. verify (#1183) and target_limit (#1184).

Related

• Tracking: implement ADR-0009, operator-typed query terms for Stack Encrypt #1178: tracks ADR-0009.
• docs(adr): ADR-0009, operators select the index and a query term is not the stored term #1177: ADR-0009.