External Lean 4 proof for MovingSofa.GerversSofa.ABφθSpec.existsUnique

I have completed an external Lean 4 formal proof of the existing theorem
`MovingSofa.GerversSofa.ABφθSpec.existsUnique` in
`FormalConjectures/Wikipedia/MovingSofa.lean`.

The external development mirrors the upstream `ABφθSpec`:
the same physical-domain inequalities and the same four equations,
and proves the exact tuple-shaped existence-and-uniqueness statement.

External proof repository:
https://github.com/dawidmtrela-dotcom/GerverSofaLean

Stable release:
https://github.com/dawidmtrela-dotcom/GerverSofaLean/releases/tag/v1.0.0

Verification summary:

- Lean 4.33.0
- final theorem builds successfully
- 8713 / 8713 global-exclusion obligations have kernel-checked proof providers
- audited local import closure: 907 Lean source files
- no `sorry`, `admit`, `native_decide`, custom `axiom`, or `unsafe`
in the final audited local source closure
- `#print axioms` reports only `propext`, `Classical.choice`, and `Quot.sound`
- full reproducibility evidence and SHA-256 manifests are included in the release

Because the proof is substantially longer than the 25–50 line guideline for
in-tree proofs, I propose keeping the proof external and submitting a small PR
that adds:

```lean
@[formal_proof using lean4 at
"https://github.com/dawidmtrela-dotcom/GerverSofaLean/releases/tag/v1.0.0"]

Start in FormalConjectures/Wikipedia/MovingSofa.lean at MovingSofa.GerversSofa.ABφθSpec.existsUnique and compare the proposed external-proof annotation with existing annotations in the repository. Verify that the v1.0.0 release URL is recorded as shown and that the file still builds successfully.

Written by the indexing model from the issue text.