Default a branch's inherit to its own name; report empty branches

[DavidSagan]

The bug

A branch's inherit is optional and "Default is the name of the Branch" (lattice-construction.md, s:lattice.construct). So

branches:
  - ln:
      periodic: false

is the branch ln built from the BeamLine ln, overriding its periodic. Expansion reached a definition only through a bare - ln or an explicit inherit:, and never consulted the entry's own key — so this form expanded to nothing at all, silently.

The missing elements then surfaced somewhere unrelated. On a real file the only sign was:

controller 'oo': target 'q>MagneticMultipoleP.Kn1' matches nothing in the expanded lattice

which is true but says nothing useful, and it also masked a genuine error in the file (a line referring to an element that did not exist).

The fix

• default_branch_inherit runs before expand() and writes a keyed branch entry's own key back out as an inherit, when the branch has neither an explicit inherit nor a line of its own and the name is defined. Appending it leaves the branch's own keys ahead of the inherited ones, which is what keeps periodic an override. Fork-built branches copy their to_line outright and never pass through here.

• check_branches_expanded runs after expansion — so root and forked branches alike have had their chance to fill in — and reports any branch holding no elements:

  • branch 'ln': no root BeamLine 'ln' is defined when the name was defaulted from the key and resolves to nothing;
  • branch 'ln': expanded to no elements otherwise. An explicit inherit naming nothing is left to expand()'s existing inherit: 'x' is not defined.

Without this, an empty branch is diagnosed only wherever some later check first trips over it.

Also

where() in pals_check.cpp now joins an element with its parameter group using the same > the parameter paths use:

element 'beginning_b1>ReferenceP': unknown parameter 'species_ref:'; did you mean 'species_ref'?

rather than element 'beginning_b1' ReferenceP: .... A group with no owning element still renders bare.

Tests

Four new cases (suite: 221 cases / 1162 assertions, all passing):

• test_expansion.cpp — a branch with no inherit picks up its named root line and keeps its periodic override; an explicit inherit still wins over the branch key; an empty branch is reported, both the undefined-root and empty-line variants.
• test_controllers.cpp — the reported shape end to end: q>MagneticMultipoleP.Kn1 resolves and drives the parameter.

Verified as real regressions by stubbing out both new calls: 3 of the 4 fail without the fix. The explicit-inherit case is a guard against the default overreaching, so it passes either way.

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