You are implementing a single task in the "fractals" Go CLI project. Work in `/work/fractals-cli`.

## Your task

Read the task brief at `/work/fractals-cli/.git/sdd/task-5-brief.md` — it contains the full text of Task 5 (Mandelbrot Algorithm). Implement exactly what it specifies, nothing more.

## Context

- Go 1.21, module `github.com/superpowers-test/fractals`, using cobra.
- Tasks 1–4 are complete: project setup, cobra CLI framework, the `internal/sierpinski` algorithm, and the sierpinski CLI subcommand. HEAD is clean.
- This task is algorithm-only (`internal/mandelbrot`). Do NOT wire up any CLI subcommand — that is Task 6.
- See `design.md` and `plan.md` in the repo root for the full specification.

## Requirements

- **TDD is required by the plan.** Write tests first, watch them fail, then implement to make them pass. The brief specifies the test cases to cover (output dimensions, point inside set, point outside set).
- Implement `Render(width, height, maxIter int, char string) []string` in `internal/mandelbrot/mandelbrot.go` per the brief's specification of the complex-plane region and the character gradient.
- Match the existing code style/conventions used in `internal/sierpinski`.
- Verify with `go test ./internal/mandelbrot/...` (and ensure `go build ./...` still succeeds).
- Commit your work with a clear message when the task is complete and tests pass.

## Reporting

Write your full report to `/work/fractals-cli/.git/sdd/task-5-report.md` (details: what you did, design decisions, test approach, anything notable for the reviewer).

Then return to me ONLY:
- Status (complete / blocked)
- Commit SHA(s)
- A one-line test summary
- Any concerns