Refactor direct dependency surface

[devmobasa]

Summary

Reduces Wayscriber’s direct dependency surface by replacing small direct dependencies with focused
local implementations and test helpers.

Direct external deps across the workspace go from 29 to 21.

Removed direct deps:

• env_logger
• async-trait
• base64
• signal-hook
• futures-util
• tempfile
• assert_cmd
• predicates

Notable Changes

• Adds local base64 encode/decode support for persisted image/session data.
• Replaces env_logger with a small local logger, including file logging and RUST_LOG filtering.
• Replaces signal-hook usage with a local Unix signal bridge.
• Adds local StreamExt::next equivalent for zbus stream polling.
• Replaces test-only tempfile, assert_cmd, and predicates usage with local helpers and direct assertions.

Signal Handling Notes

The new signal bridge preserves the important daemon behavior:

• installs handlers before publishing the daemon pid
• uses SA_RESTART
• stores pending signals outside the self-pipe so full pipes do not drop shutdown/toggle signals
• preserves errno across the signal handler
• restores already-installed handlers on partial install failure

It intentionally does not chain prior signal handlers; Wayscriber owns these daemon/overlay signals.