Epic: v4 vs v5 parity audit

Track parity between v4 and v5 across stacks and tooling. Each
sub-audit below should land as its own follow-up issue when we're
ready to schedule it; results feed into a living parity matrix doc in
the repo.

## Sub-audits

### Stacks

- [ ] .NET isolated
- [ ] Node
- [ ] Python
- [ ] PowerShell
- [ ] Java
- [ ] Go

### Tools / extensions

- [ ] Extension bundles
- [ ] KEDA / Kubernetes tooling (coordinate with M3 K8s/KEDA
investigation)
- [ ] Azure Container Apps tooling
- [ ] Custom handlers
- [ ] Worker indexing model (per-stack diffs)

## Definition of Done

- [ ] Living parity matrix doc landed in repo with update cadence +
owner
- [ ] Each sub-audit above has its own tracking issue opened
- [ ] GA-blocking gaps surfaced as implementation issues against the
right owner

Start with the sub-audit checklist in issue #5355 and identify the existing v4/v5 coverage for each listed stack and tool. Create the living parity matrix with an update cadence and owner, open a tracking issue for every sub-audit, and surface GA-blocking gaps as implementation issues for the appropriate owner.

Written by the indexing model from the issue text.