Add Nix/devenv compatibility report for Base manifests

[codeforester]

Goal

Add a Nix/devenv compatibility report for Base manifests before attempting any generated Nix output.

Related: #876

Background

Nix/devenv support should start as a compatibility bridge, not a new runtime ownership model. A report-first slice lets Base identify which manifest concepts map cleanly, which are lossy, and which should remain project-owned.

Scope

• Add a report mode for a Base manifest that classifies fields as supported, unsupported, lossy, or project-owned for Nix/devenv.
• Provide human-readable and JSON output.
• Include fixture coverage for common supported and unsupported manifest features.
• Document that this is an analysis bridge and not a generated Nix environment yet.

Acceptance Criteria

• The report is deterministic and testable.
• Each unsupported or lossy field includes a short reason.
• The command exits successfully when unsupported fields are present, unless the manifest itself is invalid.
• Documentation explains what future generated output would need before becoming safe.

Validation

• Run the focused manifest/report tests.
• Run git diff --check.

Non-Goals

• Do not generate flake.nix, devenv.nix, or similar files in this issue.
• Do not require Nix to be installed.
• Do not change Base setup semantics.