feat(plan-canvas): download artifacts as PDF

docs/testing/plan-canvas-pdf-export.tdd.md

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+# Plan Canvas PDF export TDD evidence
+
+## User journey
+
+As a Plan Canvas reviewer, I can select **Download PDF** and receive the current
+artifact as a real PDF file without sending the plan to an external converter.
+
+## Guarantees
+
+| Guarantee | Evidence | Result |
+| --- | --- | --- |
+| The Canvas exposes a labeled PDF download action | Server integration test and browser accessibility tree | PASS |
+| The browser fetches a PDF, creates a Blob URL, and starts a named download | Generated-client integration test and real Chrome interaction | PASS |
+| Only a loopback artifact URL can reach the renderer | `assertLoopbackUrl` tests | PASS |
+| Filenames are useful and safe across platforms | `pdfFileName` tests | PASS |
+| Incomplete output is never served as a PDF | `%PDF` header and `%%EOF` completion tests | PASS |
+| Renderer state is private and temporary | Fake-process lifecycle test and live process/temp-state inspection | PASS |
+| Export adds no cloud converter or npm runtime dependency | Implementation and package diff inspection | PASS |
+
+## Red and green
+
+- RED: the focused server suite produced 30 passes and 2 failures because the
+  Canvas had no Download PDF control or PDF endpoint.
+- GREEN: renderer unit tests pass 6/6, Plan Canvas server tests pass 32/32,
+  and the end-to-end review workflow passes 10/10.
+- FULL SUITE: `npm test` passes all 4,003 discovered tests; full ESLint,
+  Markdown lint, package dry-run, and `git diff --check` also pass.
+- COVERAGE: `npm run coverage` passes 4,003/4,003 with 88.97% statements,
+  80.58% branches, 94.22% functions, and 88.97% lines. The Plan Canvas
+  module group reaches 96.72% statements and lines, 84.47% branches, and
+  95.34% functions.
+- BROWSER: Chrome selected Download PDF on the real Sandbox Execution Fabric.
+  The request returned HTTP 200, `application/pdf`, and downloaded
+  `EXECUTION-FABRIC.pdf` to the browser's download directory.
+- DOCUMENT: the downloaded Sandbox PDF is a valid PDF 1.4 document with five
+  pages. The ECC 2 to ECC 3 master plan exports as a valid eight-page PDF.
+  The HTML release-preview artifact exports as a valid four-page PDF.
+  Rendered first-page previews retain headings, body text, tables/code styling,
+  and print-safe light colors.
+
+## Runtime contract
+
+The loopback server discovers Google Chrome, Chromium, or Microsoft Edge, or
+uses `ECC_PLAN_CANVAS_CHROME_PATH`. It launches the executable without a shell,
+with a private temporary profile and a loopback-only artifact URL. Completion
+requires both a `%PDF-` header and `%%EOF` marker. The renderer is terminated and
+temporary state is removed before the response is handed to the browser.
+
+If no renderer exists, the browser receives an actionable local error. Plan
+Canvas does not upload the artifact or add a hosted conversion dependency.