Track fast-uri v4 migration (currently pinned to ^3.0.1)

**What version of Ajv are you using?**
Latest (8.x)

**What problem do you want to solve?**
ajv pins `fast-uri` at `^3.0.1`. #2658 and PR #2654 track patching within the 3.x line (CVE-2026-16221, bumping to 3.1.4), but nothing tracks moving past the v3 major to fast-uri v4. Because ajv sits so deep in the dependency graph, this forces any downstream consumer that needs fast-uri v4 to carry a manual resolution/override pin instead of getting it through a normal ajv upgrade. This is the highest-impact single dependency override across our fleet — it currently affects 9 of our internal projects.

**What do you think is the correct solution to the problem?**
Open a dedicated tracking issue (separate from the 3.x patch work) for bumping to the fast-uri v4 major — acknowledging this may require a semver-major ajv release if fast-uri v4's API changed in breaking ways.

**Will you be able to implement it?**
No — flagging from the downstream/consumer side; happy to help test a release candidate.