RLSSM: audit and simplify ssms ↔ HSSM config handshake

[krishnbera]

Context

From @AlexanderFengler review on PR #278 (ssms/rl/config.py:543):

We probably need to make it an action item to do one round of cleanup on config handshakes. Doubt that we have simplified this as much as we can in terms of duplication risk.

But also have to take a look, from memory not 100% sure about how all the handshakes actually work currently.

Handshake surfaces

Surface	ssms	HSSM
Decision-process structural config	_ssm_config / ModelConfigBuilder	Config.from_defaults(decision_process)
Computed vs fixed SSM params	_resolve_handshake(), computed_param_mapping	via assembled.computed_params
Simulator theta	required_params	N/A
HSSM sampling contract	list_params, bounds, params_default	copied from assembled snapshot
Response/choice mapping	choices, response_to_choice	_ssms_response_to_choice
Context columns	context_fields	extra_fields in from_rlssm_dict
Participant trial layout	participant_contract	_make_ssms_computed_functions inputs
Structural export	to_hssm_config_dict()	from_rlssm_dict()
Executable replay	AssembledModel.assemble_participant_fn()	@annotate_function wrappers

Full handoff checklist: _local/hssm-rlssm-api/ssms-bridge-handoff.md.

Questions

• Can to_hssm_config_dict() and from_ssms_model() converge to one canonical bridge entry?
• Can participant_contract be the single source of truth for trial layout?
• Can assembled metadata drop duplication already on ModelConfig?
• Rename HSSM extra_fields ↔ ssms context_fields?
• Minor vs major API changes acceptable on either side?

Deferred from PR #278; separate PR(s).

[linear-code]

HSSM-636