Add volume() support for Spectrahedron via S3 dispatch + Rcpp stub

[Mohit-Lakra]

What this PR does

This PR adds the R-level plumbing needed for volume() to accept Spectrahedron objects, without changing the existing polytope / zonotope behavior.

Makes volume() an S3 generic and preserves existing behavior in volume.default() (so current users are unaffected).

Adds volume.Spectrahedron() with a strict verbosity flag:

0 = silent (no messages)

1 = coarse progress messages

2 = debug messages

The function validates that verbosity is a single integer in {0,1,2} and errors otherwise.

Adds a minimal Rcpp entry point volume_spectrahedra() which currently stops with a clear “not implemented yet” error (to avoid returning fabricated / incorrect numeric results).

Adds testthat coverage for:

invalid verbosity values,

message/no-message behavior controlled by verbosity,

and the expected backend stop.

Why

Spectrahedron support is being built incrementally. This PR puts the user-facing R API + native symbol + tests in place first, so follow-up PRs can implement the actual volume estimator without changing the interface again.

How to test

Run the new test file only:

testthat::test_file("tests/testthat/test_spectrahedron_volume.R", load_package = "source")

Output :-

[Mohit-Lakra]

Drafting for now, becaue currently its computing stub value, later will add functionality to compute real volume