fix: audit fixes — power timeout, timing mitigation, release profile, GREETD_SOCK cache (v0.7.1)

- Add 30s timeout with SIGKILL to power actions (adapted from moonset)
- Add 500ms minimum login response time against timing enumeration
- Cache GREETD_SOCK in GreeterState at startup
- Add [profile.release] with LTO, codegen-units=1, strip
- Add compressed="true" to GResource CSS/SVG entries
- Add SYNC comments to duplicated blur/background functions
- Add nix dependency for signal handling in power timeout

DECISIONS.md

@@ -1,5 +1,12 @@
 # Decisions
 
+## 2026-03-31 – Fourth audit: power timeout, timing mitigation, release profile, GREETD_SOCK caching
+
+- **Who**: Ragnar, Dom
+- **Why**: Fourth triple audit found moongreet power.rs had no timeout on loginctl (greeter could freeze), username enumeration via timing differential, GREETD_SOCK re-read on every login, missing release profile, and missing GResource compression.
+- **Tradeoffs**: 500ms minimum login response time adds slight delay on fast auth but prevents timing-based username enumeration. Power timeout (30s + SIGKILL) matches moonset pattern — aggressive but prevents greeter freeze.
+- **How**: (1) power.rs adapted from moonset with 30s timeout + SIGKILL (nix dependency added). (2) 500ms min response floor in attempt_login via Instant + glib::timeout_future. (3) GREETD_SOCK cached in GreeterState at startup. (4) `[profile.release]` with LTO, codegen-units=1, strip. (5) `compressed="true"` on GResource entries. (6) SYNC comments on duplicated blur/background functions.
+
 ## 2026-03-30 – Full audit fix: security, quality, performance (v0.6.2)
 
 - **Who**: Ragnar, Dom