fix(render): normalize external SVG namespaces

[miguel-heygen]

Problem

High-quality renders silently produced a visually divergent best-effort MP4 when an external SVG omitted its root xmlns. Validation and snapshots passed, but capture emitted FrameCapture:media_load_failed and still exited 0.

Originating source mapping: this PR addresses the authorized report at https://slack.com/archives/C0BGC335AQY/p1784694361653429 (candidate #1520).

Fix

• normalize namespace-less standalone SVG documents at the producer file-server boundary before browser capture
• reuse the same normalizer when the core bundler inlines SVG assets, keeping preview/snapshot and render behavior aligned
• preserve already-namespaced SVG content unchanged

Verification

• bun test packages/producer/src/services/fileServer.test.ts — 49 passed
• bunx vitest run packages/core/src/compiler/htmlBundler.test.ts — 45 passed
• bun run build — passed
• exact sanitized fixture: check 0 errors/warnings; snapshot saved 2 PNGs; high render exit 0 with no media_load_failed or correctness warning
• fixed MP4 matches the namespace control byte-for-byte: 5,818 bytes, SHA-256 8940560844427cd94a71d8cb914d0b5c21c63ee26ffe0f51270d7be2d61f38a8, SSIM All 1.000000

Numeric-ID lint and TTS/custom-score behavior are intentionally untouched.