perf: inline constant shifts and lower rotation idioms

[gbotrel]

edit: not urgent to merge, just keeping around

Found while profiling the keccak bench.

What

• A shift by a compile-time constant amount used to call a synthesized barrel-shifter module. Now it is just a split of the source register and a rejoin — no helper module, no lookups.
• (x << n) | (x >> (w - n)) is now recognised as a rotation. Constant amounts are inlined the same way; dynamic amounts call a new rotation chain, which is cheaper than two shift chains plus an OR lookup.
• Concat splitting can now borrow low bits from the next source limb when a target limb is wider than the current chunk. The odd-width limbs produced by inlined rotations need this (joining e.g. u49::u15 used to panic).

Numbers

zkc trace --stats, KOALABEAR_16:

bench	before	after
keccak (4,236-byte message)	35.9M cells	29.3M (−18%)
blake	3.9M cells	3.2M (−19%)

The new rotate_01 fixture alone drops from 11 helper modules (9 shift chain levels + 2 OR buses) to 4 rotation chain levels.

[Copilot]

Pull request overview

This PR improves ZkC VM bitwise lowering performance by (1) inlining constant shifts, (2) recognizing rotation idioms and lowering them to cheaper sequences (inline for constants; helper-chain for dynamic), and (3) enhancing concat splitting to support odd-width limb patterns produced by rotations.

Changes:

• Inline SHL/SHR when the shift amount is a compile-time constant (no barrel-shifter helpers / lookups).
• Add rotation recognition + lowering (constant rotations inline; dynamic rotations via a new rotation chain).
• Extend concat chunk formation to “borrow” low bits from subsequent source limbs when a target limb is wider than the current source chunk; add an end-to-end rotation unit fixture.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
testdata/zkc/unit/rotate_01.zkc	New unit fixture exercising variable + constant rotation idioms.
testdata/zkc/unit/rotate_01.rejects	Reject traces for rotate_01 fixture.
testdata/zkc/unit/rotate_01.accepts	Accept traces for rotate_01 fixture.
pkg/zkc/vm/internal/transform/split/concat.go	Borrow bits from following limbs to avoid mid-stream zero-padding when targets are wider than the selected source chunk.
pkg/zkc/vm/internal/transform/rotation_scan.go	New rotation-idiom scanner used to rewrite OR-of-opposite-shifts into rotations.
pkg/zkc/vm/internal/transform/rot_helpers.go	New rotation helper-chain builder (rotl/rotr) for dynamic amounts.
pkg/zkc/vm/internal/transform/lower_shift.go	Extend shift helper key to distinguish rotation helpers from shift helpers.
pkg/zkc/vm/internal/transform/lower_bitwise.go	Wire in constant scanning + rotation detection; inline constant shifts; rewrite recognized rotations.
pkg/zkc/vm/internal/transform/constant_scan.go	New constant register scanner to enable constant shift inlining.
pkg/test/zkc_unit_test.go	Register the new rotate_01 unit test.

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