docs: describe the reference data by what it is

[luisleo526]

This rewords comments, docs and READMEs that described where the reference trade lists and bars came from. They now describe the data by what it is. It changes no code, no test and no measured result.

What changed. 67 files: 42 Markdown files (fixture READMEs, docs, the root and benchmarks READMEs), 9 Python files and 16 C++ files (comments and docstrings only). Nothing under src/ or include/.

How it was checked.

• Every changed Python file has an identical token stream and AST once comments and docstrings are removed; every changed C++ file has an identical token stream once comments are removed.
• A read of the whole diff found it text-only and found no sentence that a tracked file does not support.
• Two kinds of reference tape (the date-only one, whose last row is marked Open, and the unmarked one) are named consistently throughout.

What this leaves, and why.

• Comment wording in six other files (.gitignore, CMakeLists.txt, one source file and three scripts) lands with a separate change, because they fall outside the paths a text-only change may take without a full sweep.
• benchmarks/README.md:18 still carries an older phrase. The provenance checker (benchmarks/check_provenance.py) matches that exact phrase, so the README line and the checker's pattern change together in one small follow-up change that adds a test showing the checker still rejects a false claim.
• benchmarks/results/selection.json and selection.md are generated output. The strings that produce them, and the probe identifiers they carry, change in a later change together with a regeneration, not by hand.
• Identifiers that scripts and tests read are unchanged: an environment variable and its default path, a command-line flag, a field name in the population document, probe identifiers, string literals that tests compare, and the pinned values in the benchmark selection.
• Fixture data files (metrics.json, meta.json and the like) are not touched; they belong to separate changes.
• Wording of the form "exported" in prose that does not name a method is not changed here.

The diff view will show the removed lines, and they remain in history.