Fix mesh shader OutputIndices subscript error by adding missing ref accessor (#7929)

Fixes the Slang compiler internal error "subscript had no getter" when
reading from mesh shader output index arrays (e.g., `triangles[0].x`).

## Problem
The `OutputIndices` struct was missing a `ref` accessor in its
`__subscript` implementation, causing the compiler to fail when trying
to materialize subscript expressions as r-values.

## Solution
Added the missing `ref` accessor to `OutputIndices.__subscript` using
the `kIROp_MeshOutputRef` intrinsic operation, matching the pattern used
in `OutputVertices` and `OutputPrimitives`.

## Files Changed
- `source/slang/core.meta.slang` - Added missing `ref` accessor
- `tests/bugs/gh-7925.slang` - Test case to reproduce and verify the fix

Fixes #7925

Generated with [Claude Code](https://claude.ai/code)

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Lujin Wang <lujinwangnv@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: slangbot <ellieh+slangbot@nvidia.com>
Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

0 files changed, 0 insertions, 0 deletions