Review of 3073a5061 found the remaining registration hole: Register
rejected canonical descriptor overrides but still accepted a canonical
NAME with a nil override, inheriting the canonical descriptor while
replacing the governed handler in the map - an extension could re-register
pulse_read and bypass its execution-intent enforcement.
Registration authority is now split. registerBuiltin is the unexported
construction-time path for canonical Pulse tools: shared descriptor
mandatory, overrides rejected. RegisterExtension - the only path exposed
through PulseToolExecutor.RegisterTool - rejects every canonical tool
name outright and requires the extension to declare its own descriptor.
Both paths are append-only: a name registers exactly once, so no later
registration can swap out an already-governed handler.
Proofs cover the exact bypass (canonical name, nil override), extension
and builtin duplicate rejection, builtin override rejection, and
descriptor-less extension rejection. Tests that previously swapped
handlers by re-registering now use fresh executors per scenario.
Contract prose and source pins updated.
rcourtman
8c8affc041