More flexible STDP make

[rowleya]

Refactor make process a little to allow other STDP systems which don't need the weight and timing rules to be separately implemented. This is particularly relevant to NESTML where the STDP code is generated in a single file.

[Copilot]

Pull request overview

This PR refactors the STDP build flow so timing/weight rule headers are enforced at compile time (instead of make-time), making it easier to support STDP implementations where code is generated/packaged differently (e.g., a single generated file as in NESTML).

Changes:

• Move “missing timing/weight rule” enforcement into synapse_dynamics_stdp_common.h via compile-time #error checks.
• Simplify STDP-related make rules by removing the older STDP_ENABLED / SYNGEN_ENABLED gating and building include lists incrementally.
• Minor cleanup/normalization (indentation tidy-ups; make clean also removes modified_src output).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
neural_modelling/src/neuron/plasticity/stdp/synapse_dynamics_stdp_mad_impl.c	Removes redundant post_events.h include (now pulled via common header) and normalizes indentation.
neural_modelling/src/neuron/plasticity/stdp/synapse_dynamics_stdp_common.h	Adds explicit compile-time errors if timing/weight rule APIs haven’t been included.
neural_modelling/make/synapse_build.mk	Refactors synapse-only build rules to accumulate STDP_INCLUDES/SYNGEN_INCLUDES and simplifies compile macros/clean output.
neural_modelling/make/neuron_only_build.mk	Refactors neuron include handling into a file list + pattern expansion; clean also removes modified_src.
neural_modelling/make/neural_build.mk	Mirrors the synapse build refactor for full neural builds; simplifies compile macros and clean output.

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