fix: audit LOW fixes — docs, rustdoc, scope, debug gate, lto fat (v0.6.12)

- Update CLAUDE.md and README.md to reflect the blur range [0,200] that
  the code has clamped to since v0.6.8.
- Move the // SYNC: comment above the /// doc on MAX_BLUR_DIMENSION so
  rustdoc renders one coherent paragraph instead of a truncated sentence.
- Narrow check_account visibility to pub(crate) and document the caller
  precondition (username must come from users::get_current_user()).
- Gate MOONLOCK_DEBUG behind #[cfg(debug_assertions)]. Release builds
  always run at LevelFilter::Info so a session script cannot escalate
  journal verbosity to leak fprintd / D-Bus internals.
- Document why pam_setcred is deliberately not called in authenticate().
- Release profile: lto = "fat" instead of "thin" — doubles release build
  time for better cross-crate inlining on the auth + i18n hot paths.

DECISIONS.md

@@ -2,6 +2,13 @@
 
 Architectural and design decisions for Moonlock, in reverse chronological order.
 
+## 2026-04-24 – Audit LOW fixes: docs, rustdoc, check_account scope, debug gating, lto fat (v0.6.12)
+
+- **Who**: ClaudeCode, Dom
+- **Why**: Six LOW findings cleared in a single pass. (1) Docs referenced the old `[0,100]` blur range; code clamps `[0,200]` since v0.6.8. (2) The `MAX_BLUR_DIMENSION` doc comment was split by a `// SYNC:` block, producing a truncated sentence in rustdoc. (3) `check_account` was `pub` and relied on callers only ever passing `getuid()`-derived usernames; the contract was not enforced by the type system. (4) `MOONLOCK_DEBUG` env var flipped log verbosity to Debug in release builds, letting a compromised session script escalate journal noise about fprintd / D-Bus. (5) `pam_setcred` absence was undocumented. (6) `[profile.release]` used `lto = "thin"` — fine for most crates, but for a latency-critical auth binary compiled once per release, fat LTO's extra cross-crate inlining is worth the ~1 min build hit.
+- **Tradeoffs**: `lto = "fat"` roughly doubles release build time (~30 s → ~60 s) for slightly better inlining across PAM FFI wrappers and the i18n/status paths. `#[cfg(debug_assertions)]` on the debug-level selector means you have to run a debug build to raise log level — inconvenient for live troubleshooting, but aligned with the security-first posture.
+- **How**: (1) `CLAUDE.md` + `README.md` updated to `[0,200]`. (2) `// SYNC:` block moved above the `///` doc so rustdoc renders one coherent paragraph. (3) `check_account` visibility narrowed to `pub(crate)` with a `Precondition` paragraph explaining the username contract. (4) Debug-level selection wrapped in `#[cfg(debug_assertions)]`; release builds always run at `LevelFilter::Info`. (5) Added a comment block in `authenticate()` documenting why `pam_setcred` is deliberately absent and where it would hook in if needed. (6) `lto = "fat"` in `Cargo.toml`.
+
 ## 2026-04-24 – Audit MEDIUM fixes: D-Bus cleanup race, TOCTOU open, FP reset, GTK entry clear (v0.6.11)
 
 - **Who**: ClaudeCode, Dom