perf(lsp): batch progress updates and coalesce redraws

[fcoury-oai]

Why

#293 restored LSP work-done progress, but the editor handled only one incoming LSP message per 10 ms background tick. A progress burst could therefore hold an already-available hover, completion, or diagnostic response for more than a second. Progress notifications also triggered full redraws even when Fidget was disabled.

What changed

• Drain up to 64 queued LSP messages per background tick while retaining a bounded fairness budget.
• Recognize progress notifications before plugin dispatch, so they redraw only when a listener actually changes visible UI.
• Coalesce Fidget overlay updates into the existing end-of-tick redraw.
• Add deterministic regressions for queued hover responses, numeric and string progress tokens without Fidget, the 64-message budget, and a single Fidget redraw per burst.

A controlled debug-build probe queued 100 synthetic progress updates immediately before an available hover response:

Configuration	Before	After
Fidget disabled	1,202 ms	21 ms
Fidget enabled	1,355 ms	135 ms

How to Test

1. Run the focused queueing and redraw regressions:

   cargo test -p red --lib lsp_progress
   cargo test -p red --lib fidget_progress_burst_coalesces_overlay_updates_into_one_redraw
   cargo test -p red --test lsp_lazy

   Expect queued hover responses to complete in one background pass, a 65th message to remain queued for fairness, progress without Fidget to cause no redraw, and a Fidget burst to produce one redraw.

2. Open a Rust workspace, trigger K while rust-analyzer is indexing, and confirm hover appears promptly while the Fidget overlay continues to update.

3. Add disabled_plugins = ["fidget"] to the top level of the user config, repeat the same hover request during indexing, and confirm progress-only notifications no longer repaint the editor.

Also verified: 2,077 library tests, 48 LSP integration tests, formatting, and cargo clippy --all-targets --all-features -- -D warnings.