DDIR explain: keep the user's key and value whole in demand rows

[frankmcsherry]

Explain's demand rows spliced the user's value into one flat tuple with the iteration chain and query id, (K ; V fields…, chain…, q).
That required each collection's field counts, which explain read from projection syntax, and it broke in several ways:

• A user term reading a whole row saw explain's extra fields. Reach with an always-true filter($1 == tuple($1[0])) gave an insufficient explanation, and a flatmap list reading $0 saw explain's (key, row) pair, so the rewritten program failed to install.
• A projection given as one term (value($1), and corgi lambdas to come) counted as one field whatever its shape.
• Keys and values that are neither products nor units (an int source, value($1 * 2), a collect list) were read by field position.

Change

Demand rows are now (K ; (V, chain…, q)): the user's key and value whole, then the chain, then q. No rule takes K or V apart, so explain needs no shapes or field counts.

• The syntax-based field counting and every width parameter of RowModel and the reverse rules are gone.
• User terms (filter predicates, projections, flatmap lists, join projections) read $0 as the key and $1 as the value's first field (with_rows).
• A scope's own sites are wrapped as (V) before their first lift, so the lifts give (V, chain…). Root-level host forms and sources enter the explain scope as (K ; (V)).
• Demand sets stay the user's rows, for the forward clone and the exports.

Interface

• explain(p) / explain_with(p, options) take no source shapes; source_arities and export_shape are removed.
• A query row is (key ; (val, q)).
• The server's load option is explain or explain=debug (was explain=<arity>[,debug]); README updated. dump_explain takes no arity.

explain/mod.rs goes from 1112 to 973 lines.

Tests

• whole_row_explanations_sufficient_small: a whole-row filter and a flatmap reading $0.
• scalar_explanations_sufficient_small, corgi_agrees_on_scalar_explanation: scalar source keys and values, a scalar value and a list at the root, scalar keys in an iterative scope.
• The reverse-rule contract tests are rewritten for the nested rows.
• report_demand_excess prints the same demand sizes as master-next, row for row.

cargo test -p interactive --lib --tests, explain's ignored sweeps (--ignored), and the ddir-server tests pass.

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