feat(consensus): disallow version 4 transactions at NU7

[ValarDragon]

Motivation

ZIP 2003 deprecates Sprout by disallowing version 4 transactions at NU7. The original implementation in #856 was merged into the unmerged ZIP 218 branch rather than main, so it needs a focused replacement on the current codebase.

Solution

When the off-by-default nu7-experimental feature is enabled, reject version 4 transactions from the configured network exact NU7 activation height onward. Default builds and networks without an explicit NU7 activation continue accepting V4 transactions in post-NU5 eras.

Resolve NU7 directly inside zakura-consensus, avoiding the two public zakura-chain::Network helpers added by #856. A later configured upgrade therefore cannot substitute for an omitted NU7 activation.

The build-feature prerequisite landed in #958. This branch is rebased directly onto main after that merge.

Testing

• cargo test --locked -p zakura-consensus --lib: 235 passed, 1 ignored
• cargo test --locked -p zakura-consensus --lib --features nu7-experimental: 235 passed, 1 ignored
• cargo test --locked -p zakura-consensus v4_deprecation_boundary in both build configurations
• cargo clippy --locked -p zakura-consensus --all-targets -- -D warnings
• cargo clippy --locked -p zakura-consensus --all-targets --features nu7-experimental -- -D warnings
• cargo fmt --all -- --check
• markdownlint docs/changelog/unreleased/959.md --config .github/.markdownlint.yaml
• ./scripts/changelog.py check
• git diff --check

The boundary test covers the height immediately before NU7, the activation height, a later height, the default build, the feature-enabled build, and a network that omits NU7.

Changelog

Added docs/changelog/unreleased/959.md.

Specifications & References

• ZIP 2003
• Original implementation: feat(consensus): disallow version 4 transactions at NU7 #856
• Build-feature prerequisite: build: add an experimental NU7 feature #958

Follow-up Work

None.