feat(v3/updater): add Config.OnUpdateApplied for post-update bookkeeping

[AlbinoGeek]

Description

WEP: #6203

Apps had no hook for post-update bookkeeping, such as refreshing an NSIS DisplayVersion.

Config.OnUpdateApplied(previousVersion) is called once from Init on the first launch after Restart replaced the app. The helper passes the replaced version via a temp-dir marker keyed by the target path, so it works through open -n on macOS where environment variables do not survive.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• WEP (proposal only; no implementation)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Added tests in helper_test.go and updater_test.go for the marker round trip and the once-only callback.

cd v3 && go test ./pkg/updater/... && go vet ./pkg/updater/... pass on this branch alone; gofmt -l on the changed files is empty.

• Windows
• macOS
• Linux (unit tests only; Fedora Linux 44, amd64, go1.26.x). I did not run a full update end to end on other platforms.

Test Configuration

Fedora Linux 44 Workstation, amd64, GNOME on Wayland. Only the pkg/updater unit tests were run for this change, so no wails doctor output applies.

Checklist:

• (v2 only) I have updated website/src/pages/changelog.mdx with details of this PR (v3 changelog entries are added automatically)
• My code follows the general coding style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

An entry is added to v3/UNRELEASED_CHANGELOG.md and the updater guide is updated.

This adds public API (a new optional config field, non-breaking), so it follows the WEP process: the proposal is #6203. This PR is its draft reference implementation and stays in draft until the WEP is decided.

Overlap with sibling PRs: this is one of three independent updater PRs from the same author (EXDEV staging, AppImage self-path, OnUpdateApplied). Each branch is based on master and passes on its own. They touch the same files in a few places (spawn.go, helper.go, updater.go, the guide, and the changelog), including the small resolveTarget helper that two of them also add, so whichever merges second may need a trivial textual rebase.

The code and this description were written with an AI assistant (Claude). I reviewed the diff and ran the tests listed above.

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