Test-build pipeline: fix lock scope, remove dead code, close coverage gaps

[mkarlesky]

Summary

Audit of the test-build pipeline (lib/ceedling/test_invoker/) against its own README, comments, and test coverage, following the two major refactorings that produced its current stage-sequenced architecture.

• Real bug fixed: stage_determine_files held the shared-state lock across a call to @plugin_manager.pre_test, an arbitrary plugin hook — not just the struct-field writes the README says the lock is for. Under Batchinator parallelism this serialized every test's plugin-hook invocation behind one global mutex. Moved the call outside the lock.
• Dead code removed (each confirmed via git history against a real, currently-active replacement, not guessed): Stage#run?, Testable#results_fail, Testable#no_link_objects, TestBuildPlanner#fetch_include_search_paths_for_test_file, TestBuildPlanner#form_partials_filenames.
• Doc/comment drift fixed: README's accumulator-list count (missing objects_list), a duplicate stage heading ("Collecting More Test Context" used for two different stages), a couple of typos, a misspelled method name, and generate_executable_now renamed to generate_executable (the _now suffix was a disambiguation artifact from the original Rake-extraction refactor).
• Documented, not changed: why a Partial-replaced module's own source is always compiled (stage 15) and only excluded from the objects handed to the linker (stage 16) — a context-conditional "skip the compile entirely for plain builds" optimization was tried during this work but reverted after the Docker system-spec run (delta_builds_spec.rb, wondrous_forest project) showed a plain, non-coverage build still needs that compile for its own dependency tracking to keep working.
• Test coverage added for previously-untested branches: generate_executable's ShellException rescue path, :mocking combined with mocks disabled, stage_prepare_build_paths, the mock/Partials-in-use validators, tailor_search_paths's special-case branches, and TEST_SOURCE_FILE() validation under assembly support.

Test plan

• bundle exec rspec spec/units — 2587 examples, 0 failures
• Docker (throwtheswitch/madsciencelab-plugins:v1.1.4) spec/system/delta_builds_spec.rb — 16 examples, 0 failures
• Docker spec/system/gcov_deployment_spec.rb's Partials/coverage cases — 16 examples, 0 failures

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