postprocessing: property_recorder — buffer graph properties, write CSV / time-series VTK (closes #18)

[petlenz]

Implements #18. A post-processing material that buffers named graph properties as the analysis runs and writes them out afterwards, so a run's history can be inspected without a live plotting dependency or a hand-rolled loop in every driver.

The branch predates the plasticity split and the UMAT layer; main is merged in here. The only collision was tests/CMakeLists.txt (both sides append a test registration) — none of the recorder code touched anything the split moved. 269/269 tests pass, verified locally against the merge result.

Shape

It follows the same wiring as the existing live-plot postprocessor — "material::property" sources, a dummy output so the engine calls update() each step, one add_input per source — but accumulates a record_buffer instead of pushing to a plot:

param.insert<std::string>("name", "recorder");
param.insert<std::vector<std::string>>("scalar_sources", {"solver::dgamma"});
param.insert<std::vector<std::string>>("tensor_sources", {"j2::stress"});
auto& rec = ctx.create<property_recorder<P>>(param);
ctx.finalize();
for (...) ctx.update();                    // one row buffered per step
rec.write(csv_writer{}, "history.csv");    // or vtk_timeseries_writer{}

Rank-2 tensors are flattened to Dim*Dim component columns <name>_ij. Full storage — symmetric-only reduction is deliberately left as a later option rather than guessed at.

Writing is separated from recording: record_buffer (io/record_buffer.h) holds the data, and any output_writer consumes it. CSV and time-series VTK are provided; adding a format does not touch the material.

Review round already applied

The second commit addresses ten review findings; three are worth calling out because they are the kind that survive a passing test suite:

• Locale-independent double formatting. The default stream locale can emit 0,5 for 0.5, silently producing a CSV that a reader splits into two columns. Formatting is now locale- and flag-independent, pinned by DoubleFormattingIgnoresLocaleAndFlags.
• Colliding column names are rejected. Two sources flattening to the same column header produced a file with duplicate columns and no error.
• Buffer and IO hardening on the write path.

Six tests cover the round trip, the rejections, and that the VTK output is well-formed rather than merely non-empty.

Not addressed here

property_recorder is not registered in the factory, so it cannot yet be named from a JSON document. #33 closed for the material and solver classes; this one is deliberately left for a follow-up, since a document-driven recorder wants its output path and format expressed in the deck too, and that is a design question rather than a registration line.