feat(security_guard): #347 PR 3 — Alt+Shift+W warn-event dump

[fentas]

Summary

Closes the optional polish item from #347's PR plan. Adds an explicit action + binding for surfacing the warn buffer that `WarnSubscriber` accumulates in the background.

Pieces

What	Where
New `Action.security_guard_show_warnings` variant	`src/keymap.zig`
Default binding `Alt+Shift+W` (legacy `\x1bW` + kitty kbd `\x1b[87;4u`)	`src/defaults.zig`
`WarnSubscriber.clear()` — locks, frees events, preserves `dropped_total` (audit counter)	`src/modules/security_guard/warn_subscriber.zig`
`security_guard.onAction` — snapshot, render one line per event to the sink, call `sub.clear()`	`src/modules/security_guard.zig`
Proxy switch case — dispatches to module + swallows the meta keystroke	`src/proxy.zig`

Why render-and-clear, not an interactive overlay

The issue's pinned plan proposed an interactive picker with per-event detail + dismiss-one / dismiss-all. After implementation thought:

• An alt-screen overlay would compete with vim / htop / less for the screen — atty's existing alt-screen handoff to TUIs is the whole reason `mouse_urls` / `mouse_links` are gated on `!shell_owns_input`.
• Warn events are append-only audit data, not a workflow that needs in-place mutation. "Render once + dismiss-all" preserves the scrollback record AND clears the `⚠ N` segment so the user knows they've reviewed the batch.
• New kernel events from the daemon re-arm the segment immediately — the buffer is reusable after clear.

If someone needs per-event dismiss later, the surface is small (add another action that pops a specific index) and the kernel of work — the `clear()` method, the dump format — is already in place.

Event format

```
atty security_guard: 2 warn events
. pid=

ppid= comm= argv0=
...
```

Empty buffer / no subscriber attached → one-line notice instead of silent swallow, so the keystroke is visibly registered.

Tests

• `clear()` empties the buffer but preserves `dropped_total`; buffer reusable after.
• `onAction` warn dump renders events + clears + returns consumed (with both pid=4242 and pid=4243 present in the sink output).
• Empty-buffer notice.
• No-subscriber notice (security_guard disabled).
• `onAction` returns false for unrelated actions (`incognito_toggle`, `show_help`).

Test plan

• `zig build test` — 1112/1112 (+5 new)
• `zig fmt --check`
• `zig build -Doptimize=ReleaseSafe` — 10.8 MB binary
• Manual smoke (deferred — needs daemon with `--ebpf-mode=warn` + a warn-eligible process)

🤖 Generated with Claude Code

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