chore(release): v4.3.0

[aminsammara]

Promote v4.3.0-rc.1 to stable. Single commit prepending a ## [4.3.0] entry to CHANGELOG.md; no other files touched.

What's in this PR

The 4.3.0 changelog entry is a superset of the existing 4.3.0-rc.1 entry plus four lines for the commits added between rc.1 and now:

• Bug Fixes
  • aztec: use perl -i for portable in-place edit in add_crate.sh (fix(aztec): use perl -i for portable in-place edit in add_crate.sh #23335)
  • aztec-up: fall back to no timeout when /usr/bin/timeout absent on macOS (fix(aztec-up): fall back to no timeout when /usr/bin/timeout absent (macOS) #23310)
  • release-image: stamp stdlib/package.json with release version (fix(release-image): stamp stdlib/package.json with release version #23393)
• Miscellaneous
  • reduce compat e2e timeout (chore: reduce compat e2e timeout #23318)

The ## [4.3.0-rc.1] section is preserved below the new entry for historical continuity.

Filtered from the changelog

Eight commits in v4.3.0-rc.1..HEAD are dropped as noise: the two chore: merge v4-next into v4 merge commits, the pre-PR dupe 75fd494a43 of #23393, the two umbrella chore: Accumulated backports PRs (#23386, #23321 — their contents are #23310/#23335/#23318 listed above), the internal Merge branch 'v4-next' into backport-to-v4-next-staging commit, and the v4-next manifest bump to 4.4.0 (release machinery, doesn't apply to v4).

After merging

1. Tag v4.3.0 on the merge commit and push the tag — that kicks off the release pipeline.
2. The manifest stays at 4.3.0 for the 4.3.X patch cycle; bump only if you're cutting 4.4.0 from this branch.

Test plan

• CI green
• Visual check that the 4.3.0 section reads correctly in CHANGELOG.md