refactor(security): introduce PipeName newtype (Phase 4 sub-phase 5b.1)

[githubrobbi]

Summary

Sub-phase 5b.1 of the Phase 4 newtype migration: replace the free pipe_name_for_current_user() function and bare String pipe paths with a validated PipeName newtype in uffs-security::pipe, mirroring the DriveLetter precedent from sub-phase 5b.

Pipe paths now carry their Win32 invariants in the type system:

• \\.\pipe\ prefix
• non-empty name segment
• ≤ 256 chars total (Win32 CreateNamedPipeW limit)

API surface

• PipeName::for_current_user() -> io::Result<Self> — canonical deterministic per-user path (\\.\pipe\uffs-<fnv1a64-of-user-sid>).
• PipeName::parse<Input: Into<String>>(input) -> Result<Self, PipeNameError> — validates owned-or-borrowed strings; returns a structured error enum (BadPrefix { raw } / Empty / TooLong { len }).
• as_str(), Display, AsRef<str> for FFI + tracing call-sites.

Consumers migrated

• uffs-client::daemon_ctl::pipe_name() now returns io::Result<PipeName>.
• connect_platform.rs, connect_sync_platform.rs, the keep-alive nudge in daemon_ctl.rs, and the blackhole-pipe test in windows_deadline.rs all pass name.as_str() to Win32 .open().
• uffs-daemon::ipc::create_pipe_server() takes &PipeName instead of &str, propagating the invariant to the FFI boundary.

Tests strengthened (no suppression hacks)

• Pre-existing deterministic-per-user + canonical-shape tests now exercise the typed API.
• 4 new parse-validation tests: accepts valid path, rejects missing prefix, rejects empty-after-prefix, rejects over-length input.
• Round-trip test: for_current_user() output re-parses as a valid PipeName.
• windows_deadline.rs test fixture builds its unique pipe path through PipeName::parse — doubles as a regression pin against invariant drift.

Lint posture

Full just lint-pre-push gate passes locally (fmt + clippy host + clippy Windows + lint-tests + rustdoc + doc-tests + tests + smoke + deny + manifest-audit). Zero #[allow] / #[expect] added; surgical fixes only.

Follow-ups (separate PRs)

• Sub-phase 5c: Usn newtype
• Sub-phase 5d: Frs newtype