refactor(hir): split hir.rs into data shapes + hir/lower.rs + hir/infer.rs

Pure code-movement (no logic/behavior changes). hir.rs 3,721 -> 514 (data
shapes only). Added child modules to the existing hir/ dir:

- hir/lower.rs (2,411): impl Hir + lowering free fns (build_function_bodies,
  collect_function_body_facts, lower_hir_*, task-group drains, body-fact
  collectors, sig/type-info builders).
- hir/infer.rs (815): infer_hir_expr_type + type-substitution/return-type/
  element-type helpers.

Data-shape type defs (ParamEffect..Hir struct, CallResolution, etc.) all kept
in hir.rs as the shared vocabulary. Original pub levels preserved; external
crate::hir::X paths kept via pub(crate) re-exports (infer_hir_expr_type,
assign_target_reads); 9 module-private fns widened to pub(super); tests-used
methods (fields_named, is_handle_field_name) widened. Content multiset equals
original modulo import/re-export redistribution.

Verified: cargo test -p rsscript --no-run (default + native-jit) clean; clippy
-p rsscript --tests -D warnings clean; static checker_frontend 206/0,
checker_lowering 229/0; hir::tests 14/0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: olwang