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Adds allow_backprop to the McXtrace TRACE runtime parameters, so the generated code gets #define allow_backprop (_particle->allow_backprop) (and the matching #undef), as McStas already does. Part of #118.

#247 added the allow_backprop member to the McXtrace particle struct (for McCode >= v3.6.9), but not the TRACE define. The McXtrace runtime macros (PROP_*, DISALLOW_BACKPROP, ... in mcxtrace-r.h) use the bare name, so almost every McXtrace instrument failed to compile against current McCode:

error: use of undeclared identifier 'allow_backprop'; did you mean 'mcallowbackprop'?

Classic reference: mccode/src/cogen.c.in:1705 in McCode includes allow_backprop in the McXtrace list.

Testing

• New tests/test_trace_allow_backprop.py: passes with the fix, fails without it.
• Full McXtrace mxtest --nsigma 5 --mpi=auto (macOS arm64, McCode main, mcxtrace-antlr as MCCOGEN):
  • before: 116/121 instruments fail to compile (115 on allow_backprop)
  • after: all compile and run, except 2 caused by McCode components, which are fixed in McXtrace: component fixes for mcxtrace-antlr compatibility McCode#2851. Every flagged value or display result was numerically identical under classic mcxtrace.
• Not tested: Linux, Windows, GPU.

Not in this PR (noticed while testing)

• IR cache sidecars (*.comp.<build>.json) are not invalidated when a SHARE INHERIT parent component changes (SAXSPDB kept a stale copy of SAXSPDBFast's SHARE).

🤖 Generated with Claude Code

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@g5t cool. antlr-test-path also enabled in McCode mcxtrace workflows now.