refactor: reduce cyclomatic complexity in the mesh I/O readers and mutation routines (radon D/E)

[domattioli]

Finding

A radon cc scan of src/chilmesh reports eleven functions at rank D or E (complexity 21 or higher). Ranked worst first:

Rank	Complexity	Location
E	36	gmsh_io._read_msh_v4_1
E	34	fort14_io.read_fort14_raw
E	32	_vendor_admesh_truss.distmesh2d_warmstart
D	26	fort13_io.read_fort13
D	25	gmsh_io._read_msh_v2_2
D	24	CHILmesh.direct_smoother
D	22	CHILmesh.read_from_fort14
D	22	mutations.MutableMesh.collapse_edge
D	22	summary_io._summary_from_file
D	21	mutations.MutableMesh.repeel_local
D	21	write_fort14

The pattern is consistent: the format readers each parse a header, then a nodes block, then an elements block, then an optional boundary block, all inline in one function with interleaved validation. That is why each reader also accumulates unused-variable warnings — several int(...) parses exist purely to raise ValueError for the surrounding except to convert into a GmshParseError, so the assigned name is never read.

Why this is not a mechanical fix

Splitting a reader into per-section helpers changes where exceptions originate and which partial state is visible at each failure point. Since fort.14 round-trip fidelity is a hard compatibility constraint, any split needs round-trip tests over the full fixture corpus to prove byte-identical output before and after, not just a green unit suite.

Suggested approach

Take one function at a time, highest complexity first, and for each:

1. Extract the per-section parse into a private helper that raises the same error type.
2. Give the helper direct tests, including malformed-input cases.
3. Confirm round-trip identity on all four fixtures (annulus, donut, block_o, structured) plus the fort.14 reference corpus.

_vendor_admesh_truss.distmesh2d_warmstart should be left alone — it is a vendored copy and should track upstream rather than diverge.

Scope guard

No change to _skeletonize() behavior, public API signatures, fort.14 I/O semantics, or adjacency invariants.

[model: claude-opus, repo: CHILmesh, session: dev-cleanup sweep]