refactor: require disjoint DecompressTo buffers

[jxsl13]

Follow-up to #12.

Change

DecompressTo now explicitly requires dst and data to use disjoint backing storage. Passing aliased or overlapping slices is invalid API use.

This removes the runtime overlap detector, its unsafe import and address arithmetic. It deliberately does not replace them with reflect. The overlap-specific tests are removed because overlap is no longer a supported contract.

Normal append-style buffer reuse remains unchanged and allocation-free when input and output buffers are distinct.

Rationale

Aliasing compressed input with writable output storage is a caller error and is not needed by the Teeworlds/DDNet packet path. Making that precondition explicit keeps the hot path simple, avoids architecture-dependent address inspection, and removes the exceptional source copy.

Verification

• go test ./...
• go test -race ./...
• go test -gcflags=all=-d=checkptr=2 ./...
• go vet ./...
• staticcheck ./...
• Cross-compiled test binaries for linux/386, linux/arm, and windows/386
• Full test suite passed in an actual linux/386 container
• Alternating full benchmark suite against current upstream/master: 0.52% lower geomean time; allocation counts unchanged

[ChillerDragon]

e okay? Xd