Cluster E shared scaffold: DRY the path/name disambiguation specs

[mkarlesky]

Summary

PR 2 of 5 in the system-test redesign (Stage 2 plan, following the "System Test Duplication Assessment" report's Finding 3). Pure structural refactor across the 15-file path/name disambiguation cluster — same coverage, same 44 examples, same example descriptions before and after (verified by diffing --dry-run output).

Changes

Adds three helpers to spec/support/system/spec_system_helper.rb:

• in_project(&block) — collapses the @c.with_context { Dir.chdir(@proj_name) { ... } } triple-nesting repeated at 80+ call sites across this cluster into one call.
• copy_fixture(fixture_relative_path, dest_dir) — collapses the FileUtils.mkdir_p + FileUtils.cp test_asset_path(...) pair repeated identically in the 11 test_asset_path-based files.
• RSpec.shared_context "a fresh ceedling gem project", proj_prefix — collapses the verbatim before(:all)/after(:all)/@proj_name/ceedling new scaffold sequence, identical across all 15 files, into one include_context line.

Also extracts copy_duplicate_dup_pairs — a byte-for-byte identical private method that existed independently in both implicit_source_header_correspondence_disambiguation_spec.rb and test_source_file_directive_spec.rb — into one shared helper both files call.

sibling_header_collision_spec.rb and bracket_path_handling_spec.rb keep their inline C-fixture content untouched, per the plan's investigation (collapsing to shared fixtures would scatter documentation the inline content carries, or trade a runtime-generated bracket-named directory for a more fragile checked-in one) — only their outer scaffolding skeleton changes.

16 files changed, 774 insertions(+), 1180 deletions(-) — net -406 lines.

Verification

• All 15 changed files, --dry-run: identical 44 examples, identical descriptions before/after.
• All 15 changed files, real run: 44/44 pass, host (macOS) and the madsciencelab-plugins Docker image (Linux).
• sibling_header_collision_spec.rb's transform was scripted (regex-based skeleton unwrap) and diffed line-by-line against the pre-transform file to confirm only re-indentation and the intended skeleton lines changed — no C-fixture content drift (squiggly heredocs are indentation-independent regardless).
• Full-suite validation (all 16 CI matrix jobs) is CI's job per the Stage 2 plan's process — not duplicated locally.

This is PR 2 of 5; the plan calls for a go-ahead before starting PR 3 (Cluster A battery trim).

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