Orbit: Bypass handling for residual connections

[edwjchen]

Stacks on #3194.

A residual connection is a fork that feeds a deep "main" path and a shallow path which rejoin at an addition. No single value is live across that span, so the region has no single-input single-output cut and SISO partitioning would leave it as one monolithic ILP. When the two rejoined operands differ in multiplicative depth by at least bypass-depth-threshold (Orbit's bpsdepth, default 15), the region is instead split and solved separately.

How it works

• Detection (detectBypass): finds the fork/join in a region, e.g., an output addition with two secret operands from distinct producers whose mul-depth gap clears the threshold.
• Split solve (solveBypass): the deep main path is solved for every enumerated output level to build a QBP table; the shallow path plus the join is then solved once per realized main-output state under the region's own output boundary; the cheapest main + bypass combination is kept.
• Runs on every region (solveRegion dispatcher)
• Residual-join carve-out: the join addition is kept out of any squashed addition tree (via per-op multiplicative depth in OpGrouping) so its two-predecessor structure stays visible to detection.
• Contested fork: the fork is pinned to the region's DP-enumerated input state and each path rescales it independently per edge, so a rescale common to both paths is charged once (absorbed into the previous partition's output) rather than duplicated.

To make bypass regions first-class, the partition driver was generalized from a contiguous [begin, end) range to an explicit op-set (groupIds), so main and bypass can be arbitrary subsets of a region.

Default

On by default at threshold 15, matching Orbit; set bypass-depth-threshold=0 to disable. Existing circuits without a large-gap residual are unaffected.