Collapse the actor-stamping factories onto the shared core

[xmap]

Stacked on #605. Base is fix/presence-names-the-caller, not main, because this depends on the actor_kwarg knob that PR adds. Review or merge #605 first.

What

Agent, Subject and Budget each carried a full copy of make_update_handler's body, and each said in its own docstring that the copy existed because the core could not thread principal_id to the decider. #605 added actor_kwarg to the core and removed that reason, so all three become thin wrappers like their non-stamping siblings.

Their _ActorUpdateHandler protocols go with the bodies: they existed only to describe a return shape the core already declares.

4 files changed, 96 insertions(+), 409 deletions(-)

Federation does not collapse

For a second and unrelated reason, so it stays. The Seal aggregate is a per-facility singleton whose stream id derives from command.facility_code rather than being read off a command attribute, so that factory also carries resolve_stream_id, which the core has no equivalent for.

Collapsing it means adding a second knob to the shared core, which is its own decision with its own blast radius. The file now records that, with the trigger being a second aggregate anywhere needing a derived stream id.

Envelope coverage consolidates rather than disappearing

Worth stating explicitly, because the shape of this change is also the shape of a lost check.

test_envelope_principal_id asserts each handler stamps principal_id onto the event envelope, and skips files that delegate, because the assertion is made once at the delegate instead. Three cases move from passed to skipped:

before   138 passed, 175 skipped
after    135 passed, 178 skipped

The delta is exactly the three factories collapsed here, and the cora.infrastructure.update_handler case passes.

Verification

ruff + pyright     clean
unit + contract    16,118 passed, 1 skipped
architecture       30,105 passed, 618 skipped
pre-commit hooks   all passed

🤖 Generated with Claude Code

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
apps/api/src/cora/agent
_agent_update_handler.py
apps/api/src/cora/budget
_allocation_update_handler.py
apps/api/src/cora/federation
_actor_update_handler.py
apps/api/src/cora/subject
_subject_update_handler.py
Project Total

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