expressions: tree-general index deduction (Phase E) — inner-node general products

[evaleev]

Stacked on #562. Completes the general-product story for arbitrary expression trees: general products (fused + contracted + free indices) no longer need to sit at the root of their own assignment.

What

• Top-down index-set deduction (MultEngine::init_indices(target)): an index shared by a product's two children is fused iff the node's target carries it, contracted at the node otherwise; an index neither the sibling nor the target carries is consumed entirely within the child subtree and is dropped from its demand (pure intersection). The up-pass is the new available_indices() (per-subtree leaf-annotation union, valid before init); each child dictates the ORDER of its demand via preferred_layout() (canonical (fused, left-free, right-free) for products). Expressions consumed without a target (reductions) retain the bottom-up contraction convention and remain unsupported for inner general products.
• Streaming result re-permute for general products (replaces the interleaved-target gate): a target differing from the canonical result layout cannot be folded into the batched tile op (BatchedContractReduce must stay perm-free), so the product evaluates canonically (perm-free batched Summa over a slab-replicated pmap) and a UnaryEvalImpl wrapper re-permutes to the consumer's layout. Honors TA's implicit-permute contract: when the consumer fuses the permutation into its transposed GEMM (implicit_permute_outer), only tile ordinals/trange are remapped and contents stay canonical. This also lifts the former root-level interleaved-target limitation.

With this, the THC reconstruction evaluates in ONE expression:

g("p,q,r,s") = x("p,r1") * x("q,r1") * z("r1,r2") * x("r,r2") * x("s,r2");

(left-deep tree; r1 is fused where the two x factors meet, contracted where z joins, and dropped from demands above), verified against the explicit-intermediates staging.

Tests

• New: THC one-expression vs explicit intermediates; minimal depth-2 (general feeding contraction, exercises the implicit-permute path); root-level non-canonical target (exercises the content-permuting wrapper); scratch deduction unit checks.
• The former expression_general_product_inner_node_gated throw test is superseded (the expression now evaluates).
• Full regression: general_product, einsum_*, expressions{,_sparse} (modulo the two pre-existing assign_subblock_block_base1 failures), tot_contraction/permutations — green.
• mpqc c6h14/cc-pVDZ PNO-CCSD energy unchanged (3e-11, run-to-run noise level).

Out of scope (follow-ups)

• ScalMultEngine (scaled products) keeps depth-1 semantics for inner general products.
• ToT inner-index result permutations remain gated.
• Mixed T*ToT in arbitrary trees (Phase F) — to be scoped separately.