test(make-pdf): diagram pre-pass unit suite + e2e render gates

34 unit tests (fence extraction incl. nested/tilde/unclosed/render=false,
info-string parsing, slot substitution, diagnostic/figure escaping + SVG
script strip, byte-level dimension probing across 5 formats, content-box
math, image inlining incl. strict/remote/missing/data-URI paths). E2E gate
proves through the compiled binary: both fences render as vector text
(id-collision check), raw mermaid ships only via render=false, broken fence
yields the diagnostic block, and the relative fixture image rasterizes to
colored pixels (CRITICAL regression for the about:blank image fix).
--strict exits non-zero on a missing image.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

make-pdf/test/fixtures/diagram-gate.md

@@ -0,0 +1,37 @@
+# Diagram Gate
+
+A relative local image (CRITICAL regression: must render, not 404):
+
+![a red box](./diagram-assets/red-box.png)
+
+## First diagram
+
+```mermaid title="Gate pipeline"
+graph LR
+  GATEALPHA[gatealphanode] --> GATEBETA{gatebetanode}
+  GATEBETA -->|yes| GATEGAMMA[gategammanode]
+```
+
+## Second diagram (id-collision check)
+
+```mermaid
+graph TD
+  GATEDELTA[gatedeltanode] --> GATEEPSILON[gateepsilonnode]
+```
+
+## Kept as source
+
+```mermaid render=false
+graph LR
+  RAWKEPT --> ASCODE
+```
+
+## Deliberately broken
+
+```mermaid
+graph LR
+  A -->
+  (((
+```
+
+Done.