[TypeSpec Authoring] Support description and code alignment assessment

## Goal

Add an assessment for the TypeSpec `@doc` decorator on changed public API declarations.

## Scope

- Check whether a changed or newly added public declaration has an applicable `@doc`.
- Detect missing, empty, placeholder, or clearly outdated `@doc` content.
- Check that `@doc` still matches the declaration after changes such as renames, type changes, optionality changes, defaults, or behavior changes.
- Report the affected declaration, the issue with its `@doc`, and an exact TypeSpec source link.
- Avoid duplicate findings when one shared declaration is used by multiple operations.

This task is limited to assessing `@doc`. Generated documentation and examples are tracked separately.

## Acceptance criteria

- Missing or empty `@doc` on changed public declarations is detected.
- Clearly stale `@doc` that contradicts the changed declaration is detected.
- Existing applicable `@doc` does not produce a finding.
- Every finding links to the relevant TypeSpec source.
- Tests cover missing, empty, stale, valid, shared-declaration, and no-impact cases.

The issue names no implementation files, tests, or entry points. Start by locating the TypeSpec authoring assessment code and its existing tests, then use the acceptance criteria to verify missing, empty, stale, valid, shared-declaration, and no-impact cases, including exact source links and duplicate suppression.

Written by the indexing model from the issue text.