fix(multi): Suppress VGitSync during staging; 95%+ performance improvement

[MaxFangX]

Builds on top of #422 and #423.

Description

Fixes performance degradation during staging operations by suppressing redundant VGitSync events. Achieves 99.6% reduction in buffer refresh calls (1,380 -> 6 LiveGutter:fetch calls) and 95% reduction in total git operations (10,421 -> 564 calls across 41 staging operations).

When staging hunks in the diff view, DiffScreen already manually fetches and renders the current buffer, making VGitSync's refresh of all tracked buffers unnecessary and wasteful. Every staging operation (s, S, u, U, r, R keys in diff view) triggered a cascade: git index changes -> filesystem watcher detects change -> VGitSync event fires -> git_buffer_store.for_each() iterates all tracked buffers -> each buffer runs LiveGutter:fetch() -> GitBuffer:diff() -> 3-4 git commands per buffer. With 15 tracked buffers, this meant 15x unnecessary buffer refreshes per stage operation, resulting in thousands of redundant git command calls.

This adds suppress_sync_for(ms) API to git_buffer_store that temporarily disables VGitSync events. Applied this 200ms suppression window to all DiffScreen staging methods (stage_hunk, unstage_hunk, reset_hunk, stage, unstage, reset), preventing the cascade while DiffScreen handles its own refresh. Also includes with_selective_staging() API for future use cases where only specific buffers should refresh.

Tested with 15 tracked buffers over 41 staging operations. No functional regressions observed - all git operations complete successfully, gutter signs update correctly, and UI remains responsive.

Performance Summary

Improvement:

• Total calls: 95% reduction (10,421 -> 564)
• LiveGutter:fetch: 99.6% reduction (1,380 -> 6)
• GitBuffer:diff: 99.6% reduction (1,381 -> 6)
• git ls-files: 94% reduction (1,463 -> 87)

Profiling Details

Environment: 15 tracked buffers, ~40 stage operations (DiffScreen:stage_hunk)

Before:

Unique operations: 36
Total calls: 10,421

156054ms total (1380x, 113ms avg) - vgit.features.buffer.LiveGutter:fetch
102827ms total (1381x, 74ms avg) - vgit.git.GitBuffer:diff
69420ms total (1838x, 38ms avg) - vgit.git.git_buffer_store.dispatch
41300ms total (1463x, 28ms avg) - gitcli.run: git ls-files
20863ms total (768x, 27ms avg) - gitcli.run: git ls-files -u
18357ms total (468x, 39ms avg) - gitcli.run: git show
11166ms total (39x, 286ms avg) - vgit.features.screens.DiffScreen:stage_hunk

After:

Unique operations: 21
Total calls: 564

6502ms total (41x, 159ms avg) - vgit.features.screens.DiffScreen:stage_hunk
1410ms total (87x, 16ms avg) - gitcli.run: git ls-files
723ms total (38x, 19ms avg) - vgit.git.git_stager.stage_hunk
339ms total (6x, 57ms avg) - vgit.git.git_buffer_store.dispatch
242ms total (6x, 40ms avg) - vgit.features.buffer.LiveGutter:fetch
241ms total (6x, 40ms avg) - vgit.git.GitBuffer:diff