Autodetect application-bundled FFmpeg runtimes

[mcuadros]

Summary

• add FFMPEG_LIBRARY_DIR as the highest-priority desktop runtime override
• discover coherent FFmpeg families beside the executable and in conventional lib/ffmpeg subdirectories
• prefer the directory of the selected FFmpeg runtime when locating ffshim
• preserve Android linker-namespace and iOS framework behavior
• regenerate the guarded shim contract and document the new lookup rules

Validation

• go test -count=1 ./internal/bindings ./internal/shim
• go test -count=1 -skip ^TestDecoderAutoHardwareLifecycle$ ./...
• go vet ./...
• go test -race -count=1 ./internal/bindings ./internal/shim
• go run ./internal/shim/cmd/gencontract -check
• CGO-disabled short suite
• cross-compilation gates for Windows amd64/arm64 and macOS amd64/arm64

Known master issue

The unmodified master branch also fails TestDecoderAutoHardwareLifecycle on this GPU host because first-time driver initialization grows the process FD count from 6 to 17. That pre-existing failure is not changed or hidden in this PR; the remainder of the suite passes when only that test is excluded.

Release note

The shim implementation is contract-hashed, so this change intentionally regenerates the contract identifier. The next release must publish matching shim assets; v1.0.0 shims remain paired with v1.0.0.