Introduce ShellClass (Application | System) as a session-scoped
classification derived from PostureLevel at ceremony grant time.
- ShellClass::Application: default, software operations only
- ShellClass::System: host operations, requires Normal (5) posture
- derive_shell_class(): pure function, configurable threshold
- satisfies(): hierarchical check (System satisfies Application)
- No mid-session upgrade by design (immutable in SessionScope)
Added shell_class and posture_level_at_establishment to SessionScope
with #[serde(default)] for backward-compatible deserialization.
Signed-off-by: Tyler King <tking@guildhouse.dev>
Signed-off-by: Tyler J King <tking727@gmail.com>