docs(mxc): correct IsolationSession filesystem support

[prekshivyas]

Summary

Correct the Windows MXC documentation to match the filesystem capability observed in v0.0.37 at a749bc88: isolation_session does not support OpenShell filesystem-policy grants, and MXC rejects non-empty read-only/read-write grants during provisioning.

PR #3473 correctly moved the shipped inference demos to process_container, but the general MXC README, architecture overview, sample gateway configuration, and Rust API comments still described isolation_session as a grant-only filesystem compatibility mode. That contradiction led QA to retest an unsupported backend/policy combination and report NVBug 6782874 as still present.

This PR changes documentation and comments only. It does not change runtime behavior.

Related Issue

• NVBug 6782874
• Follow-up to fix(mxc): repair Windows inference demos (NVBug 6782874) #3473
• QA observation on v0.0.37 / a749bc88: filesystem policy is not supported by the isolation session backend

The GTL bundle's separate PACKAGE-README.md is not tracked in this repository and must be synchronized by the package owner.

Changes

• Mark OpenShell filesystem-policy grants unsupported on isolation_session in the canonical Windows backend matrix.
• State that process_container is the filesystem-policy-capable backend and supplies the default-deny boundary.
• Clarify that isolation_session uses backend-default filesystem visibility when no grants are present.
• Align the MXC driver README, sample gateway configuration, and Rust API comments with that contract.
• Remove all stale “grant-only filesystem” claims from the Windows/MXC source tree.

Testing

Passed:

• Fern documentation check with Fern 5.112.0: 0 errors
• mise run markdown:lint:md: 139 files, 0 errors
• cargo fmt --all -- --check
• uv run --no-project python scripts/update_license_headers.py --check
• git diff --check

mise run pre-commit reached and passed the changed Markdown/Rust formatting surfaces, but the aggregate remains host-blocked while building unchanged grpcio-tools 1.78.0 under Python 3.14 on Windows ARM64 because MSVC receives incompatible /std:c++17 and /std:c11 flags.

No runtime tests were required because this PR changes documentation and comments only.

Checklist

• Targets the current windows tip
• Uses the behavior reproduced by QA as the source of truth
• Keeps the change focused on documentation accuracy
• Updates the canonical architecture page and crate-specific documentation
• Commit is DCO-signed