perf(pipeline): pre-convert ordered.Map before onContentTransformed hook dispatch

[zeroedin]

Problem

fireContentTransformedHooks (build.go:1813) passes page.FrontMatter directly into the hook payload at line 1820. When json.Marshal serializes this payload for the QuickJS/WASM bridge, any *ordered.Map values in FrontMatter trigger ordered.Map.MarshalJSON — which is 13.7% of total CPU (2.28s) due to byte-by-byte appendCompact copying.

*ordered.Map values enter FrontMatter when the cascade merges JSON data file values into the page context.

Profiling evidence (from plans/CONTENT_TRANSFORMHOOK_PERF.md)

ordered.Map.MarshalJSON   2.28s   13.7% of CPU
json.Marshal (total)      2.75s   16.6% of CPU
Inter-pass hooks total   11.9s   61.9% of build

Fix

Before building the hook payload, convert *ordered.Map values in page.FrontMatter to plain map[string]interface{} via ToGoMap(). Regular map marshaling is dramatically faster — no custom MarshalJSON, no appendCompact.

// build.go:1815 — before payload construction
fm := convertOrderedMaps(page.FrontMatter)
payload := map[string]interface{}{
    "html":        string(page.RenderedBody),
    "toc":         serializeTOC(page.TOC),
    "path":        page.RelPath,
    "url":         page.URL,
    "frontMatter": fm,  // plain maps, fast marshal
}

convertOrderedMaps recursively walks the map and calls ToGoMap() on any *ordered.Map value. This is a read-only conversion at the hook boundary — page.FrontMatter is untouched. Templates still see the original *ordered.Map values for ordered iteration.

Scope

• Only affects fireContentTransformedHooks in build.go
• Only the JSON payload sent to the QuickJS/WASM VM is converted
• Template engines (Liquid, Go templates) are not involved — they receive page.FrontMatter directly after hooks complete
• page.FrontMatter is NOT mutated — only the serialization copy is converted
• Plugins don't need key ordering in front matter — they're processing HTML and metadata

Expected impact

ordered.Map.MarshalJSON drops from 13.7% to 0%. json.Marshal total drops from ~2.75s to ~0.47s. Inter-pass hooks drop from ~12s to ~9s (remaining cost is WASM memory overhead, addressed separately).

Related

• plans/CONTENT_TRANSFORMHOOK_PERF.md — full analysis
• feat(data): ordered map for JSON data file key preservation #455 — ordered.Map merge (introduced MarshalJSON)
• fix(pipeline): expand onContentTransformed payload to page object #450 — expanded onContentTransformed payload

[zeroedin]

Closing — superseded by #465 (replace JSON serialization with native qjs.ToJsValue). If json.Marshal is eliminated entirely, pre-converting ordered.Map is unnecessary.

[zeroedin]

Reopening — #463 is still needed even with #465. qjs.ToJsValue uses reflection and cannot access *ordered.Map's unexported fields. The convertOrderedMaps step is required before either json.Marshal (current) or ToJsValue (#465) can process the payload. These are complementary: #463 handles the data conversion, #465 handles the serialization mechanism.