refactor(viewer): batch history rows before live DOM insertion

[seonghobae]

Scope

Build history rows in a DocumentFragment and publish that fragment to #history-body through one live-DOM appendChild call per render. The mock DOM implements fragment-flattening semantics so the existing executable JS suite can exercise the same publication shape.

This PR establishes a structural batching contract only. It does not claim a measured browser p95/paint/reflow improvement yet: the generated PR description inferred performance from implementation shape without browser profiling, realistic history-size distribution, or current-head E2E timings.

Review → repair

The generated test updates only made DocumentFragment available to the mock. They did not prove that multiple rows cross the live history-body boundary in one operation, so a regression could restore row-by-row publication while the tests still passed.

Normal descendant e4400a392a99a56bc6dc7b70226af2109af47047 strengthens demo-integration.test.mjs with two completed history records and instruments the actual history-body.appendChild boundary. Acceptance is now:

• both rows remain present and preserve the existing inert-text/action behavior;
• exactly one live history-body.appendChild occurs for the render;
• fragment children are flattened by the mock, matching the DOM-facing shape expected by production code.

The implementation/test history was not destructively rewritten to manufacture a historical RED. The original generated implementation preceded this regression; the descendant adds the missing executable contract and records that limitation honestly.

Exact current authority

• protected/base: main@06633a25109c62e24a7015ae04fb9f6e0a246f7e
• exact head: e4400a392a99a56bc6dc7b70226af2109af47047
• lifecycle: open / Draft / mergeable
• CI 33940807694: queued
• Security Scan 33940807668: queued
• SAST Semgrep 33940807693: queued
• CodeQL PR 33940807678: queued
• fuzz 33940807714: queued

Queued/predecessor evidence is non-passing. Promotion additionally requires the repository-authoritative mvn -B --no-transfer-progress verify on this exact head and browser profiling/E2E evidence before any buyer-visible performance claim is added to docs, CHANGELOG, or release material.

UI Delivery Gate

• 의도성: PASS — batching exists to reduce live-DOM publication frequency, not as decorative complexity.
• 기능 완전성: PARTIAL — executable structural regression exists; exact-head hosted/browser execution is pending.
• 콘텐츠 적합성: PASS — no new UI section or copy is introduced.
• 복원력: PARTIAL — existing inert-text/action behavior remains in the integration contract; real-browser responsive/a11y execution is pending.
• 증거성: PARTIAL — one-live-append behavior is testable, but performance gain is deliberately unclaimed until measured.
• 고유성: N/A — no visual identity change.

No self-approval, bypass, force update, destructive rebase, dummy rerun commit, sample-size manipulation, or synthetic performance claim.

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