Truncate hash gadget outputs with component_truncate

[HDauven]

Resolves #304

Stacked on #305.

HashGadget::finalize_truncated and HashGadget::digest_truncated now truncate with component_truncate::<250> instead of append_logic_xor::<125>(_, ZERO). The outputs stay the low 250 bits, and each truncated output takes 146 fewer gates:

Gadget	append_logic_xor	component_truncate
digest_truncated, 1 / 2 / 4 inputs	886 / 887 / 889	740 / 741 / 743
Merkle4 truncated digest	889	743

Plain digests are unchanged (652 / 653 / 655 gates), and the pinned truncated count in hash_gadget_constraint_counts moves from 889 to 743. test_truncated_gadget checks inside a circuit that the gadget output equals the native truncation, and plonk's soundness suite covers component_truncate at this width.

[Copilot]

🟢 Approval recommended

The focused substitution preserves documented semantics and is covered by the existing circuit-to-native equivalence test.

0 open findings

What changed in this PR

Replaces the hash gadget’s bitwise truncation with dusk-plonk’s lower-cost truncation component while preserving the low 250-bit output.

Changes:

• Use component_truncate::<250> for truncated gadget outputs.
• Document the gate-count optimization in the changelog.

File	Description
src/​hash/​gadget.rs	Uses the dedicated truncation component.
CHANGELOG.md	Records the optimization and issue reference.

🧠 Review effort: Balanced

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[moCello]

APPROVE — The new truncation constrains the same low 250 bits that the native Hash::finalize_truncated mask keeps, and test_truncated_gadget compares the two inside a circuit.

[HDauven]

Force-pushed 499be20, rebased onto the updated #305: the pinned truncated count moves from 889 to 743.