feat(planning): measure a vendored Mermaid renderer so the owner can decide its size

[kyle-sexton]

Problem

The page shows Mermaid source with "not available in this version" (plugins/planning/surface/index.html:1121). Rendering needs a vendored renderer. The page CSP allows no network source (test_exporters.py:416), so any renderer must be inlined into the page. The owner's decision on #4653 is to "decide Mermaid renderer size"; this issue is not the decision.

Evidence

• index.html:1121: source shown, not rendered.
• test_exporters.py:416: CSP with no network source.

Proposed approach

1. Vendor a Mermaid renderer build in a scratch location (not committed) and measure it: raw bytes, gzip bytes, and the resulting size of index.html if inlined.
2. Post the numbers on this issue, with the current index.html size for comparison.
3. Stop. The owner decides whether that size is acceptable. Do not choose a size threshold or build the integration.

Acceptance criteria

• Measured raw and gzip size of a vendored Mermaid renderer is posted on this issue.
• The owner records accept or reject; only then is an integration issue filed.

Context

Refs #4653 (owner decision 2026-09-29, Option B). The size call stays with the owner, hence status: needs-decision.

[kyle-sexton]

Resolution (2026-09-29, owner-approved consensus)

Resolution: do-here (3/3 votes). Owner approved.

Goal: post the size measurement of a vendored Mermaid renderer next to the current planning surface page (acceptance criterion 1), then stop. No integration, no threshold.

Steps for the executing agent

1. Download the pinned build into a scratch dir, not committed: https://cdn.jsdelivr.net/npm/mermaid@12.0.0/dist/mermaid.min.js.
2. Record raw bytes and gzip -9 bytes. Consensus run measured 5,575,485 raw and 1,595,955 gzip. mermaid.tiny.js does not exist in 12.0.0 (jsdelivr: "Couldn't find the requested file").
3. Measure plugins/planning/surface/index.html on origin/main. Consensus run measured 139,135 raw and 40,650 gzip.
4. Compute the inlined size: about 5.71 MB raw, about 1.64 MB gzip, roughly 41x the page.
5. Post the numbers as a comment on this issue with the mermaid version and the exact commands.
6. Cite the current line for the "not available in this version" text. The index.html:1121 reference in the issue is stale (line 1121 on origin/main is function restoreFocus(f){).

Acceptance criteria

• A comment on this issue lists raw and gzip bytes for the mermaid build and the current page, the inlined total, the mermaid version, and the commands used.
• No threshold is chosen and no integration is built.
• Owner accept or reject of the size is a separate, second criterion and stays open after the measurement is posted.

No credentials beyond existing gh auth are needed.