fix(renderer): pass through non-Nunjucks content on parse error

[e3322-pcsk9]

Problem

shardmind update crashes with RENDER_TEMPLATE_ERROR when the vault contains non-template files with literal {{ strings (e.g., test fixtures like "garbage{{" in .claude/scripts/tests/stop-checklist.test.ts).

The differ (source/core/differ.ts) renders both old and new template content through renderString() for three-way merge comparison. When Nunjucks encounters {{ that isn't a valid variable expression, it throws expected variable end.

Root Cause

renderTemplate() in source/core/renderer.ts treats every Nunjucks parse failure as fatal. Files that aren't Nunjucks templates (no {{ expr }}, {% block %}, or %} syntax) should pass through unchanged rather than crash.

Fix

In renderTemplate(), before throwing RENDER_TEMPLATE_ERROR, check if the source contains any actual Nunjucks syntax. If not, return the raw source — the parse error is from accidental {{ in non-template content, not a template authoring mistake.

function renderTemplate(source, context, env, filePath): string {
  try {
    return env.renderString(source, context);
  } catch (err) {
    if (!containsNunjucksSyntax(source)) {
      return source;
    }
    throw new ShardMindError(...);
  }
}

function containsNunjucksSyntax(source: string): boolean {
  return /%\}|{%|{{.*?}}/.test(source);
}

Test Plan

• RED: Two regression tests proving renderString crashes on "garbage{{" and "data{{"
• GREEN: Fix makes both tests pass
• Regression: All 1114 existing tests pass (78 test files)

Commits

1. test(renderer): add regression tests for literal {{ in non-template content — failing tests
2. fix(renderer): pass through non-Nunjucks content on parse error — minimal fix

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Ultraworked with Sisyphus

[breferrari]

Thanks for this — you surfaced a real crash in shardmind update that wasn't on our radar, and the diagnosis (the differ renders content that isn't a template) is spot on. I dug into the root cause and opened #132 with the full writeup.

One thing the renderer-level approach here runs into: in v6 the merge renders every modified file, but only .njk files are actually templates — everything else is copied verbatim. So the deeper fix is to stop the merge from rendering copy-origin files at all (merge their raw bytes), rather than catching the parse error in renderTemplate. Two concrete issues with catching it in the renderer:

1. It still substitutes a valid {{ user_name }} that a copy file legitimately contains (e.g. test data) — renderString succeeds and replaces it, so that path isn't caught.
2. A genuine .njk template typo like {{ user_name } (missing brace) also throws expected variable end, and the regex doesn't match it — so it would get swallowed and the broken literal emitted into the user's vault, instead of failing loudly.

I've put up #133 taking the merge-layer approach (closes #132), with your report credited in the issue, the PR, and the changelog. Would love your eyes on it — and thank you again for catching this.