Release v0.0.1-rc4

[emersonian]

Release candidate v0.0.1-rc4, built as a clean fast-forward on top of rc3 (75d57bd). Nine commits, no history rewrite.

What's in it

• Add regtest end-to-end harness and Network enum — new regtest-harness/ (own workspace) + Network enum plumbing.
• Make sync pause responsive, and speed up zkv list
• Memoize the GUI sidebar summary to avoid a per-poll replay.
• Bump crossbeam-epoch to 0.9.20 for RUSTSEC-2026-0204.
• Paint the GUI sidebar instantly on launch (names first).
• Add Ironwood (NU6.3) support — shallow scan of ironwood_actions V6 notes; recover self-send memos across pool codes 3/4; create the ironwood checkpoints table on open; trial-decrypt both external + internal scopes.
• Bump librustzcash/shardtree pins for the Ironwood scan model.
• sync: initialize the block-cache schema in run_sync (self-healing cache).
• Bump version to v0.0.1-rc4.

Purpose

Gate the release on CI (fmt, clippy -D warnings, tests, deny, tsc, regtest E2E) before tagging.

Merging — please do NOT use the green button

This branch carries curated commits (author == committer, real authored dates). GitHub's merge/squash/rebase buttons all rewrite or collapse them. When CI is green, land it as a fast-forward instead:

git push origin main   # origin/main is an ancestor, so this fast-forwards

GitHub will auto-close this PR as Merged once the commits reach main. Then push the tag:

git tag v0.0.1-rc4 476ebac && git push origin v0.0.1-rc4   # triggers release.yml