chore(release): v4.3.0 (#23405)

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
(#23335)
- `aztec-up:` fall back to no timeout when /usr/bin/timeout absent on
macOS (#23310)
- `release-image:` stamp stdlib/package.json with release version
(#23393)
- **Miscellaneous**
  - 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`

Author: aminsammara