Fix #369: drop noexcept from allocating paths

[petlenz]

Closes #369.

noexcept on paths that can allocate turned std::bad_alloc into std::terminate. Audited every noexcept in include/ and src/ (287 sites).

Removed (can allocate or throw):

• expression::operator==, !=, <, like_term_of (+ n_ary_tree, tensor_scalar_mul overrides), equals_same_type / less_than_same_type (+ visitor_base overrides): all compute child hashes lazily.
• update_hash_value overrides that hash children: n_ary_tree (also allocates a vector), unary_op, binary_op, ternary_op and their update_hash functors, tensor_scalar_mul, tensor_pow, permute_indices_wrapper, inner_product_wrapper, outer_product_wrapper, tensor_eigenprojection, tensor_eigenvector, tensor_isotropic_function, tensor_inner_product_to_scalar, tensor_to_scalar_divided_difference, tensor_to_scalar_eigenvalue, tensor_to_scalar_scalar_wrapper. (Signature lines only; Fix #372: reset m_hash_value before recomputing in every hash override #431 edits some of these bodies.)
• n_ary_tree: copy and forwarding ctors. n_ary_vector: default ctor (reserve), copy and forwarding ctors, push_back, reserve, insert_hash.
• symbol_base(name, ...) (copies the name), get_scalar_zero/one (static init allocates), symbol_table::has (builds a std::string).
• Comparators scalar_expr_less, scalar_pretty_printer; printer_base::print_sequence, latex_printer_base::print_unary; structural_propagation::preserve_unary (copies a space); tensor_data_make_imp::evaluate_imp (make_unique); the tensor-with-scalar and tensor-with-t2s mul dispatch functions (build expressions).

Moves that copied — now genuine moves, noexcept kept: tensor_add, tensor_mul, simple_outer_product move the space instead of copying it (dropping noexcept there trips clang-tidy performance-noexcept-move-constructor); permute_indices_wrapper moved via static_cast<base> (a copy), now static_cast<base &&>; scalar move-assignment moves the name.

Kept: move ctors/assignments that only move members, getters, is_same, set_coeff (shared_ptr assignment), set_space/clear_space, scalar_constant::update_hash_value (hashes a scalar_number, no children), scalar_number::abs, printer_base::begin/end and tensor_data::print (stream insertion), fixed-size tmech evaluate_imps, index arithmetic helpers, conditional noexcept(noexcept(tag_invoke(...))) CPOs, and the never-instantiated static_assert evaluator fallbacks.

Tests — CoreBugFix.AllocatingPathsAreNotNoexcept:

• static_asserts that ==, <, like_term_of, n_ary_vector::push_back and get_scalar_zero() are not noexcept; that scalar_add, tensor_add, tensor_to_scalar_mul, permute_indices_wrapper stay nothrow-move-constructible.
• Runtime: moving a tensor_add keeps its Symmetric space.

Negative control: compiling the test against main's sources fails on the five "not noexcept" asserts; removing the space transfer from tensor_add's move ctor fails the runtime check.

Full suite 2347/2347 (gcc-14 Debug); clang-tidy-18 clean on the changed .cpp files; clang-format-18 clean.

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Review fix (eb5ce92). The forwarding move ctors n_ary_tree(n_ary_tree&&, Args&&...) and n_ary_vector(n_ary_vector&&, Args&&...) kept unconditional noexcept while their copy twins lost it — same over-promise shape, since Args is forwarded into base_t. Both are now noexcept(std::is_nothrow_constructible_v<base_t, Args...>). To keep that condition true for the tensor nodes, tensor_expression(dim, rank) and tensor_expression(tensor_expression&&, dim, rank) are marked noexcept — they only store two size_t and default-init the remaining members. Plain move ctors stay unconditionally noexcept; added static_asserts for scalar_mul, tensor_mul, tensor_to_scalar_add and both expression_holder instantiations, so container nothrow-move is pinned.

Two observations left as-is (out of scope): scalar_expr_less in numsim_cas_type_traits.h is dead code — expr_ordered_map is std::map<T, T> with the default std::less, and the struct has no other reference; and scalar::operator=(scalar&&) has no callers (scalar also pulls in using base_t::operator=).

Full suite 2347/2347 after the review fix; clang-tidy-18 and clang-format-18 clean.