Rework BookProcessor to use OffScreenBrowser (BL-16430)

[JohnThomson]

Allows us to finally retire RunJavascriptWithStringResult_Sync_Dangerous.

Devin review

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This change is 

[greptile-apps]

Greptile Summary

This PR migrates BookProcessor from driving a per-page WebView2Browser on the UI thread (using the dangerous RunJavascriptWithStringResult_Sync_Dangerous / Application.DoEvents loop) to driving a single OffScreenBrowser on its own dedicated STA thread, enabling the job to run entirely on a Task.Run background thread and keeping the UI responsive throughout.

• OffScreenBrowser is extended with StartFreshBrowser() (fresh renderer, warm shared environment between pages), NavigateWithoutWaitingForLoad(), and RunJavascriptFireAndForget(), all safely serialized through RunAndBlock.
• ExternalApi.HandleProcessBook switches from BeginInvoke-on-UI-thread to Task.Run; WinForms state reconciliation is marshaled back via a new InvokeOnUiThread helper; _processBookInProgress is marked volatile to handle the now-cross-thread false write.
• RunJavascriptWithStringResult_Sync_Dangerous is fully removed from WebView2Browser and the Browser base contract, along with its Obsolete annotation.

Important Files Changed

Filename	Overview
src/BloomExe/Book/BookProcessor.cs	Reworked to use OffScreenBrowser instead of creating per-page WebView2Browser controls on the UI thread; removes the Debug.Assert UI-thread check, switches to blocking calls, and makes WaitForJavascriptResult internal (enabling unit tests). Logic is correct.
src/BloomExe/Publish/OffScreenBrowser.cs	Gains StartFreshBrowser(), NavigateWithoutWaitingForLoad(), RunJavascriptFireAndForget(), a shared CoreWebView2Environment field, and CreateWithInjectedEnvironment plumbing; the dedicated-STA-thread model is preserved and the new code serializes correctly through RunAndBlock.
src/BloomExe/web/controllers/ExternalApi.cs	Switches RunProcessBookJob from BeginInvoke-on-UI-thread to Task.Run on a background thread; adds InvokeOnUiThread helper for WinForms state updates; marks _processBookInProgress volatile to handle the now-cross-thread false write. Previous reviewer concern addressed.
src/BloomExe/WebView2Browser.cs	Removes RunJavascriptWithStringResult_Sync_Dangerous and the obsolete annotation; adds CreateWithInjectedEnvironment factory and CoreEnvironment property to support OffScreenBrowser's shared-environment approach.
src/BloomExe/IBrowser.cs	Removes the obsolete RunJavascriptWithStringResult_Sync_Dangerous abstract member; RunJavascriptFireAndForget already existed in this contract and is unchanged.
src/BloomTests/Book/BookProcessorTests.cs	New test class covering WaitForJavascriptResult (page-load value, delayed fire-and-forget value, timeout), and StartFreshBrowser isolation; marked SkipOnTeamCity as the tests require a real WebView2 environment.

_{Reviews (2): Last reviewed commit: "Post-review fixes (BL-16430 part C)" | Re-trigger Greptile}

[hatton]

@JohnThomson please address or resolve https://app.devin.ai/review/BloomBooks/BloomDesktop/pull/8038