fix(clippy): rename deprecated fetch_update to try_update

[MichaelTaylor3d]

Rust 1.99.0 (2026-09-28) renames AtomicUsize::fetch_update to try_update for consistency with the standard library. This PR updates the two deprecated calls in dig-node-core.

Refs DIG-Network/dig_ecosystem#3442
Run: 36951759927

[MichaelTaylor3d]

PASS - security audit, #633 (ticket dig_ecosystem#3442)

Head audited: 773ae94

• Diff is 2 one-line renames of AtomicX::fetch_update to try_update. Both argument orderings stay (SeqCst, SeqCst), the closures are untouched, and the surrounding Ok/Err handling is untouched. try_update is the stabilised successor with the same (set_order, fetch_order, f) signature and Result<T, T> return.
• Both sites are inside #[cfg(test)] mod tests (coinset_resolver.rs:332-333, line 418; module_reshare.rs:1000-1001, line 1625). They are a mock resolver's answers-left counter and a mock downloader's budget. No production guard, rate limit, once-flag or reshare budget is touched. The cfg(test) code is not compiled into release binaries.
• No remaining fetch_update in crates/.
• Findings: none (no LIVE issue, no defence-in-depth item).

Not covered: toolchain behaviour of try_update itself (std, assumed per its documented identical semantics); gitnexus/socraticode were unavailable and not needed for a two-line diff.

[MichaelTaylor3d]

Verdict: PASS at head 773ae94 (reviewed read-only; no code edited).

Criteria checked:

1. Semantics identical. Read core/src/sync/atomic.rs of the installed stable toolchain (rustc 1.98.1): fetch_update is a literal alias, #[deprecated(since = "1.99.0", note = "renamed to try_update")], whose body is self.try_update(set_order, fetch_order, f). Both have signature (set_order, fetch_order, f) -> Result<T, T> with Ok(previous) / Err(current). Same argument order, same return meaning.
2. Orderings unchanged at both sites: coinset_resolver.rs:418 and module_reshare.rs:1625 stay SeqCst, SeqCst; closures untouched. Diff is exactly +1/-1 per file.
3. Toolchain admits it: no rust-toolchain file; CI uses dtolnay/rust-toolchain@stable and rustup toolchain install stable. try_update is #[stable(since = "1.95.0")], and the 1.99 clippy that fails #632 is newer. The only rust-version in the repo is 1.75.0 in crates/dig-chat-protocol, which has no fetch_update or try_update, so it does not apply. Both changed sites are in mod tests of dig-node-core.
4. No remaining uses: grep -rn fetch_update over the whole tree at this head (target, .git, node_modules excluded) returns zero hits.
5. Checks: all 13 checks on #633 pass, including Clippy, Rustfmt, Test + coverage and CodeQL.

Not run: a local build under rust 1.99 (installed toolchain is 1.98.1, which compiles try_update; the CI Clippy pass covers the lint). Test-vacuity gate does not apply: a lint-only rename that changes no behaviour, so there is no fix to revert.

Threads: none opened, none open.