fix(dom,net): resolve Windows local asset deadlock and guarantee ResourceHandler drop safety

[GoCoder7]

Closes #958

Hi everyone,

1. Summary of Changes

This pull request resolves a render-blocking deadlock on Windows where loading local stylesheets or assets via compile-time absolute paths (e.g. asset!("browser.css")) leaves the application window permanently solid white (#FFFFFF).

Root Causes Addressed:

1. Windows Drive Paths Parsed as URL Schemes: BaseDocument::resolve_url previously passed Windows absolute paths (e.g. D:\...\browser.css) directly to url::Url::parse(), which interpreted the drive letter as a scheme (url.scheme() == "d").
2. Silently Dropped Resource Handlers: When blitz-net failed on scheme "d", it dropped the ResourceHandler without sending a response. Because ResourceHandler lacked a Drop implementation, pending_critical_resources.remove(...) was never called, permanently blocking layout and paint passes (render_skipped=true).
3. Improper File URL Extraction: blitz-net used request.url.path() instead of to_file_path() for file:// URLs, which fails on Windows absolute drive letters.

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2. Exact Scope & Seams Touched

Crate	File	Changes	Description
blitz-dom	packages/blitz-dom/src/document.rs	+19 / 0	Detect Windows absolute paths in resolve_url and convert to valid file:/// URLs via Url::from_file_path.
blitz-dom	packages/blitz-dom/src/net.rs	+62 / -2	Implement Drop for ResourceHandler<T> with an AtomicBool guard to guarantee that dropped handlers emit an error ResourceLoadResponse and request a redraw, ensuring pending_critical_resources always clears. Added unit tests.
blitz-net	packages/blitz-net/src/lib.rs	+6 / -1	Use request.url.to_file_path() instead of request.url.path() for file:// requests on Windows.

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3. Direct Empirical Proof (Two-Case Comparison)

To confirm that this deadlock is an independent root cause in Blitz, a controlled dual-run verification was executed on Windows hardware with the GPU backend held strictly constant (DirectX 12 enabled via upstream AnyRender PR #100):

Observation / Metric	Case A: Original Blitz + DX12 Backend	Case B: Fixed Blitz + DX12 Backend
resolved_url in <head>	'd:\business\...' (Scheme "d")	'file:///D:/business/...' (Scheme "file")
pending_critical_resources	Stuck at 2 forever	Cleared from 2 -> 0
load_resource invocations	0 (handler dropped silently)	2 (both stylesheets completed)
DOM resolve()	early_return=true (aborted)	Proceeded to normal layout
blitz-shell::redraw	render_skipped=true (aborted)	render_skipped=false (rendered)
Visual Window Output	Solid White (#FFFFFF)	Full Browser UI Rendered Crisply

This confirms that even with a fully working DX12 graphics backend, original Blitz alone is 100% sufficient to cause a permanent blank white window on Windows.

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4. Separation from Upstream AnyRender

This PR is strictly confined to the Blitz repository's asset and document lifecycle:

• The GPU swapchain presentation fix lives upstream in dioxuslabs/anyrender (Issue #99, PR #100).
• This PR contains zero graphics backend or wgpu instance selection logic, ensuring a clean, orthogonal review.

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5. Validation Summary

1. Unit Tests Added:
   • dropped_handler_emits_error_response: Verifies that dropping an unfulfilled ResourceHandler automatically emits an error DocumentEvent::ResourceLoad.
   • responded_handler_does_not_emit_on_drop: Verifies that a handler that has already responded does not emit a duplicate response on drop.
   • Run via cargo test -p blitz-dom --lib net::tests (Passed cleanly).
2. End-to-End Browser Run:
   • cargo run -rp browser launches on Windows and displays the new tab page, address bar, Blitz logo SVG, and search input box without freezing.

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Thanks!