MermaidImageRenderer: OFFICECLI_MERMAID_JS local asset override for render=image

[hesong12]

Files: src/officecli/Core/Diagram/MermaidImageRenderer.cs (ResolveMermaidJsRef(),
RefreshCacheIfPresent()), README.md

Summary

diagram --prop render=image's chrome-family backend renders with the real
mermaid.js. ResolveMermaidJsRef() resolves the script it points the page
at in three steps: an existing local cache → download from OfficeCLI's own
mirror → download from the jsDelivr CDN → give up and reference the CDN URL
live in the page. Every one of those needs a network path out. A host that
is offline, air-gapped, or sandboxed away from the network has no way to use
render=image at all, even when it already has a valid mermaid.min.js on
disk.

This PR adds OFFICECLI_MERMAID_JS: when set to a path that exists,
ResolveMermaidJsRef() returns a file:// URI for it directly — no cache
check, no download attempt, no network call of any kind — and
RefreshCacheIfPresent() (the daily background revalidation hook) returns
immediately instead of trying to reach the mirror. Unset, behaviour is
unchanged.

private static string? MermaidJsOverride
{
    get
    {
        var p = Environment.GetEnvironmentVariable("OFFICECLI_MERMAID_JS");
        return !string.IsNullOrWhiteSpace(p) && File.Exists(p) ? p : null;
    }
}

Same shape as the existing OFFICECLI_MMDC override two members away in the
same file.

Why

Same host as the sibling OFFICECLI_BROWSER PR: a desktop app that runs
inside a locked-down sandbox (network egress not guaranteed, no ability to
silently phone a CDN on the user's behalf) wants render=image diagrams to
work from an asset it ships and pins itself.

Validation

Toolchain: same scratch .NET 10 SDK as the sibling PRs.
mermaid.min.js downloaded once from
https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.min.js into a scratch
directory (not committed, not part of the diff).

Precedence check (does the override actually get read, not just
"happen to work because the network was up anyway"): point
OFFICECLI_MERMAID_JS at a deliberately invalid JS file
(not_a_valid_mermaid_js_file();) and diagram add the same source on both
the unmodified main branch and this branch, network fully reachable on
both runs:

officecli create t.docx && officecli close t.docx

# BEFORE (unmodified main — ResolveMermaidJsRef doesn't know the var exists):
OFFICECLI_MERMAID_JS=/path/to/corrupt.js officecli add t.docx / \
  --type diagram --prop mermaid=$'erDiagram\n  A ||--o{ B : has' --prop render=image --json
# → {"success":true,"data":"Added diagram at /body/p[@paraId=00100000]", ...}
#   (ignores the var, downloads the real mermaid.js from the mirror/CDN, renders fine — 18.8s)

# AFTER (this branch):
OFFICECLI_MERMAID_JS=/path/to/corrupt.js officecli add t.docx / \
  --type diagram --prop mermaid=$'erDiagram\n  A ||--o{ B : has' --prop render=image --json
# → {"success":false,"error":{"error":"mermaid failed to render: mermaid is not defined",
#     "code":"internal_error"}}
#   (loaded the corrupt local file instead of downloading — proves the override took effect — 2.2s)

Offline functional check — the real command from the task, with the
valid local copy and the network genuinely unreachable
(http_proxy/https_proxy pointed at a closed port; a raw HttpClient
probe against the same target confirms this fails in <0.1s rather than
silently succeeding):

officecli create t.docx && officecli close t.docx
OFFICECLI_MERMAID_JS=/path/to/mermaid.min.js officecli add t.docx / \
  --type diagram --prop mermaid=$'erDiagram\n  A ||--o{ B : has' --prop render=image --json
# → {"success":true,"data":"Added diagram at /body/p[@paraId=00100000]", ...}   (7.6s)
officecli view t.docx outline
# → File: t.docx | 1 paragraphs | 0 tables | 1 images
officecli view t.docx screenshot -o t.png --json
# → PNG written, 21717 bytes, real ER diagram (A —has→ B, crow's-foot notation) — see U2-screenshot.png
ls ~/.officecli/cache/     # → No such file or directory: RefreshCacheIfPresent() never touched it

Compatibility

Additive only. The existing cache → mirror → CDN → live-CDN cascade in
ResolveMermaidJsRef() is untouched and still runs whenever
OFFICECLI_MERMAID_JS is unset. RefreshCacheIfPresent()'s existing
early-return (!File.Exists(CachedJsPath)) is untouched; the new check is
strictly earlier in the same method and only short-circuits when the new var
is set.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Honor the local override in the styled ESM path

When a diagram specifies theme, layout, or look, RenderViaChrome selects BuildHtmlEsm and never calls this resolver; that path still imports MermaidEsmUrl and, for ELK, ElkEsmUrl from jsDelivr. Consequently, on the offline or sandboxed hosts this override targets, styled render=image requests still fail—or auto mode falls back to native rendering and loses the requested styling—despite OFFICECLI_MERMAID_JS being set.

Useful? React with 👍 / 👎.