Delete src/lib (ITensorsExtensions + BaseExtensions submodules) - #364

Cross-codebase usage audit found ~110 of the submodule's 258 lines
uncalled. Three groupings of changes:

**Dead-code deletions:**

- `sqrt_diag` / `inv_diag` / `invsqrt_diag` / `pinv_diag` / `pinvsqrt_diag`
  (five convenience wrappers around `map_diag`, zero callers).
- `diagblocks` (both `Tensor` and `ITensor` methods, zero callers) along
  with its supporting `DenseTensor` overloads of `NDTensors.blockoffsets`
  / `nzblocks` / `blockdim` (only consumer was the now-dead `diagblocks`).
- `LinearAlgebra.isdiag(::ITensor)` (zero callers; `apply.jl` and the
  internal `map_eigvals` reference resolve to `ITensors.isdiag`).
- `is_delta` (three methods, zero callers).
- `ITensors._contract(::Key)` dispatch patch (bypassed because
  `contract.jl:18` unwraps `Key`-wrapped sequences via `deepmap` before
  reaching `_contract`).
- `ITensors.replaceinds(::ITensor, ::Dict{<:Index, <:Index})` performance
  patch (zero callers in src/, test/, or ext/).

**Type-system block transitively dead — entry chain `trivial_space(x)`
→ `promote_indtypeof(x)` had only test callers:**

- `make_bosonic` and the `using_auto_fermion` / `has_fermionic_subspaces`
  fermion branch in `map_eigvals`. The corresponding "Fermionic map
  eigvals tests" testset in `test_itensorsextensions.jl` is dropped.
  The fermion-agnostic "Fermionic eigendecomp" testset (which tests
  `eigendecomp` directly) stays.
- `promote_indtypeof(::AbstractITensorNetwork)`, `promote_indtypeof
  (::AbstractIndsNetwork)`, and the `function promote_indtypeof end`
  declaration.
- `indtype(::ITensor)`, `indtype(::IndsNetwork)`, and `indtype(::Type{
  <:IndsNetwork{V, I}})`. The user-facing `indtype(link_space,
  site_space)` 4-form (used by the `IndsNetwork{V,I}` constructor to
  infer `I`) stays.
- `promote_indtype(::Vararg{Type{<:Index}})`, `promote_indtype_rule`,
  `promote_spacetype_rule` (four methods), `spacetype` — all only
  reached through the deleted `promote_indtypeof` chain.
- `trivial_space` (all four methods) — replaced at its single test
  callsite by an inline `hasqns`-conditional helper in `test/utils.jl`.

**Test-only relocations:**

- `replace_vertices` and its internal `set_sites` helper, used only by
  `test/test_opsum_to_ttn_mpo_cross_check.jl`, move to `test/utils.jl`.
- `group_terms`, used only by `test/test_tebd.jl`, moves to
  `test/utils.jl`.
- `src/lib/ITensorsExtensions/src/opsum.jl` is deleted outright.

**Doc cleanup:**

- Drop the `promote_indtypeof(is::AbstractIndsNetwork)` and `indtype
  (inds_network::IndsNetwork)` entries in `docs/src/experimental_methods
  .md`.

Final state: the submodule keeps `tensor_sum`, `eigendecomp` +
`map_eigvals` (now fermion-free), and the focused type-system chain
supporting the `IndsNetwork{V,I}` constructor — `IsIndexSpace`,
`indtype(link_space, site_space)` family, `indtype(space)`, `_indtype`.
About 60 LOC instead of 258.

After this collapse, src/lib/ goes away entirely. Each helper now lives
in the single src/ or test/ file that uses it:

- `eigendecomp` and `map_eigvals` move from `lib/ITensorsExtensions/`
  into `src/apply.jl` (only consumer in `src/` was `simple_update_bp`).
  Both are now module-level `ITensorNetworks` functions; `src/apply.jl`
  drops the `ITensorsExtensions.` qualifier on its four call sites.

- `tensor_sum` moves from `lib/ITensorsExtensions/` into `src/opsum.jl`
  (only consumer was `ITensors.ITensor(::Sum, ::IndsNetwork)` in the
  same file).

- The `indtype(link_space, site_space)` 4-form, `indtype(space)`,
  `_indtype` family, and `IsIndexSpace` move from
  `lib/ITensorsExtensions/` into `src/indsnetwork.jl` (only consumer
  was the `IndsNetwork{V, I}(g, link_space, site_space)` constructor).

- `maybe_real` moves from `lib/BaseExtensions/` into `src/opsum.jl`
  (consumers were `src/opsum.jl` and `src/apply.jl`; both files reach
  it as a top-level `ITensorNetworks` function now). `to_tuple` was
  only used in `test/utils.jl::group_terms` and moves there directly.

- `test/test_itensorsextensions.jl` renamed to `test/test_map_eigvals.jl`,
  imports updated from `ITensorNetworks.ITensorsExtensions: ...` to
  `ITensorNetworks: ...`. The testset name changes from
  `"ITensorsExtensions"` to `"map_eigvals"`.

- `include("lib/BaseExtensions/...")` removed from
  `src/ITensorNetworks.jl`; `src/lib/` directory deleted outright.

Net effect: drops two submodules' worth of indirection without losing
any in-tree behavior. All tests pass.

These three methods in `src/apply.jl` had no callers anywhere — TEBD
calls `apply(::Vector{ITensor}, ψ)` after materializing the Trotter
product into ITensors via `Vector{ITensor}(::Prod, ::IndsNetwork)`,
not `apply(::Op|::Prod|::Scaled, ψ)`. The rest of the OpSum→ITensor
chain in `src/opsum.jl` (including `tensor_sum` and `maybe_real`) is
still alive through that materialization step and stays.