jc: as_chunks_mut in the two fingerprint builders (clears JC Substrate Proof on main)

[AdaWorldAPI]

Why

JC Substrate Proof (jc-proof.yml) has been red on every push to main since at least 2026-09-22, including 9b6acb58. The failure is lint clippy::chunks_exact_to_as_chunks, which is new in 1.98, and it fires in crates/jc. jc is workspace-excluded, so the workspace-scoped pre-bump sweep never checked it. The issue was already recorded as TD-JC-CLIPPY-RED-ON-BASE-2, which spelled out this fix.

Change

• jirak.rs deterministic_fingerprint and pearl.rs fingerprint now iterate fp.as_chunks_mut::<8>().0 and write each [u8; 8] directly. The bytes are the same as before, because chunks_exact_mut also ignored any remainder.
• TECH_DEBT.md: the entry is marked RESOLVED for jc only. Its census of 116 ungated sites in other excluded crates stays open.

Verification (local, toolchain 1.98.1)

• Unfixed tree: the lint reproduces under --all-targets -D warnings.
• Fixed tree, running every step of the workflow: cargo test (141 + 17 passed), clippy clean, prove_it exits 0, substrate_compare exits 0. cargo fmt --check is clean.

Not related to #1270/#1272. The other non-green check on main, member-tests, shows all 56 test results ok and was cancelled during cache save. It passed on c47698fa.

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Reviewing files that changed from the base of the PR and between 9b6acb5 and 6a5e9a3.

📒 Files selected for processing (3)

• .claude/board/TECH_DEBT.md
• crates/jc/src/jirak.rs
• crates/jc/src/pearl.rs

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