Preserve explicitly empty tmux options

[barlevalon]

Summary

• resolve session and global tmux options by show-option exit status, so explicitly empty values override global values and built-in fallbacks
• preserve trailing spaces in icon and scroll-padding values
• remove eager @nowplaying_* default initialization from plugin loading, avoiding global tmux option pollution while retaining lazy runtime defaults
• keep integer validation and clamping behavior for empty or invalid numeric values

Tests

• added scope-aware mock coverage for absent, empty, precedence, trailing-space, and invalid numeric cases
• added isolated real-tmux coverage using a unique server for local/global resolution, empty icon and padding overrides, plugin-load state, override preservation, and status interpolation
• ran Bash syntax checks, the full shell suite, and ShellCheck

Residual risks

• isolated real-tmux tests ran on tmux 3.7b; tmux 2.9 behavior should remain covered by the compatibility CI matrix or an oldest-version container
• defaults written by older plugin versions remain in existing tmux servers until users unset them; removing initialization prevents new writes but cannot safely identify historical defaults

CI integration

• installs tmux in the macOS job so the new isolated real-tmux checks run under system Bash 3.2