Debounce search buffer updates during feed fetches

[benthamite]

Summary

• Debounce elfeed-search-update so that rapid feed completions coalesce into a single redraw instead of one per feed
• Fix an argument mismatch where elfeed-update-hooks passed the feed URL as the force parameter, bypassing the staleness guard and forcing a full redraw on every callback
• Add user-customizable elfeed-search-update-delay (default 1.5s)

Closes #293. Also addresses #317 and #474.

Problem

When elfeed-update fetches feeds, each completion runs:

(run-hook-with-args 'elfeed-update-hooks url)

elfeed-search-update is on this hook and receives the URL string as its optional force argument. Since any string is truthy, every feed completion triggers a forced full redraw of the search buffer (full DB scan via with-elfeed-db-visit, erase-buffer, re-render all matching entries). With N feeds, this produces N forced full redraws fired back-to-back as run-at-time 0 timer events, each blocking the main thread.

With ~300 feeds, this freezes Emacs intermittently for 30-60 seconds.

Fix

All changes are in elfeed-search.el:

1. Debounce non-forced calls. elfeed-search-update now dispatches: when force is non-nil it redraws immediately (preserving existing behavior for interactive g, filter changes, mode init); when force is nil it schedules a trailing-edge debounce timer. If another call arrives before the timer fires, it resets. This coalesces hundreds of rapid callbacks into a handful of redraws.

2. Fix the argument mismatch. A new wrapper elfeed-search--update-debounced discards the URL argument from the hook and calls elfeed-search-update with no arguments, entering the debounce path instead of the forced path. This replaces elfeed-search-update on elfeed-update-hooks.

3. Timer cleanup. The debounce timer is canceled in elfeed-search--unload when the search buffer is killed.

Result

With 317 feeds: ~5-10 redraws instead of 317+. Emacs remains fully responsive throughout the update. Interactive refresh (g), filter changes, and live filtering are unaffected.

Backward compatibility

• elfeed-update-hooks contract unchanged: hook still fires once per feed completion
• elfeed-search-update-hook unchanged: still fires after every actual buffer update
• elfeed-search-update still accepts optional force with same semantics
• elfeed-search-update--force still forces immediate update
• Custom elfeed-search-print-entry-function implementations are unaffected

Test plan

• Byte-compiles cleanly (zero new warnings)
• All 39 existing tests pass
• G in search buffer: feeds fetch without freezing, buffer updates after completions settle
• g in search buffer: immediate forced refresh (no debounce)
• Live filter (s): still updates immediately as you type
• elfeed-search-set-filter: immediate update