Add type annotations to top-level functions in ecPhlConseq

Introduce hi_arg type alias for (proofterm option * form) option and
annotate all t_hi_conseq_* dispatchers, helpers (t_hi_apply_r,
t_hi_error, pp_f_node, pp_opt_f), and the main t_hi_conseq dispatcher
with explicit argument and return types.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: strub