Warn about config-local setting overrides

[helly25]

Warn when one logical configuration section overrides its own setting, while preserving the existing last-value-wins result. This catches likely copy/paste leftovers without interfering with deliberate command-line overrides, config-tier precedence, expression repetition, or options that accumulate.

AG;DR

• Add registry-backed repetition semantics for every global option:
  • overriding settings use the default last-value-wins behavior;
  • accumulating settings such as --exclude, --summary, and --config stack without a notice;
  • keyed settings such as --define=NAME=VALUE and inline --pack-option=NAME=VALUE accumulate different names but report a repeated name.
• Add LookupGlobalArgument to canonicalize full valued, aliased, sign-suffixed, attached-short, and compatibility spellings to their global registry entry; make IsKnownGlobal use that same lookup.
• Inspect automatic system and user config plus explicitly selected xffrc files, grouping repeated physical lines by their logical selector section.
• Treat positive and negative forms, and canonical names and aliases, as one overriding setting.
• Respect primary arity, variadic command argument runs, and --, so expression matchers/actions and their arguments are never mistaken for global settings.
• Emit a warning for a section-local override and retain both values in resolution order, preserving the established last-value-wins behavior.
• Do not inspect command-line repetition. Overrides between config sections, files, tiers, and the CLI remain intentional precedence boundaries.
• Keep structurally unique config-only policy controls under their existing hard-error validator.
• Document the diagnostic in generated config help and regenerate XFF.md.
• Update the 0.4.0 changelog for this diagnostic and add the previously omitted compare-path-encoding and mixed-policy-class fixes found by auditing every commit since v0.3.7.
• Address CI clang-tidy's hicpp-use-emplace finding in the keyed-setting test.

Validation:

• bazel test //... --config=clang
• bazel test //xff/cli:config_validation_test //xff/cli:globals_test //xff/cli:explain_test //xff/cli:xff_markdown_test --config=xff_docs --config=clang
• bazel test //xff/cli:config_validation_test --config=clang after the clang-tidy fix
• focused pre-commit checks for all changed files
• git diff --check
• Clang-tidy is intentionally left to CI because the current local machine configuration cannot sustain it.