docs: address review feedback on rule testing design

Revise the design document based on Evan's PR review:

- Rewrite abstract to acknowledge existing tools (mindev rtst,
  rules_test.go) and state the gap as 'no standardized way'
- Add Prior Art section covering mindev rtst and rules_test.go
- Fix datasources as Evaluate-phase, not Ingest-phase
- Generalize REST ingest to provider-agnostic API surface
- Add brief remediate/alert definitions (out of scope for v1)
- Convert ASCII diagrams to mermaid
- Pick decisions for all open questions:
  - Test declaration: test_* function discovery
  - File layout: flat co-location
  - Datasource mocking: HTTP-level (Approach B)
  - Datasource discovery: explicit in test file
  - Check helpers: via Starlark load() statement
- Move rejected approaches to Alternatives Considered section
- Remove Go implementation code (DD6 reduced to interface table)
- Remove binary name discussion
- Formalize tone throughout
- Link to datasource documentation
- Add load() support for shared test libraries

Signed-off-by: Krrish Biswas <krrish175@gmail.com>
Signed-off-by: krrish175-byte <krrishbiswas175@gmail.com>

Author: krrish175-byte