Replace all 9 kernel uid==0 privilege checks with a capability bitmask
model. Adds caps:u64 field to Context and CallerCtx, with CAP_ALL for
root processes. Zero behavioral change - uid==0 still gets all caps.
New module: src/scheme/caps.rs with 10 capability constants.
9 check sites converted: acpi, irq, memory, debug, serio, sys (msr+write),
scheme registration, and fchown.
Patch: local/patches/kernel/P27-capability-bitmask.patch
vasilito
ce9ff8aebd