refactor: factor duplicated parse pipeline and remove dead code

[henryiii]

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Verified simplifications and dead-code removals with no intended behavior changes. Full test suite (24 tests) passes.

• Factor the parse-completion callback pipeline (App_inl.hpp): the priority-ordered _process_callbacks/_process_help_flags sequence appeared nearly verbatim in three places (the _parse(std::vector&) subcommand path, the dot-notation subcommand path, and the config section-close path). Extracted into a new private helper App::_process_completion_callbacks(bool with_help_flags). The config path is preserved exactly by passing with_help_flags=false, which also omits _process_env() and passes the matching run_callback(false, false) (vs. run_callback(false, true) for the help-flag paths). Diffed the three copies character by character first.
• Dedup parse() prologue: the identical clear/validate/configure prologue shared by parse(std::vector&) and the rvalue overload is now a small App::_parse_setup() helper. Additionally, the rvalue _parse now checks the return value of _parse_single and breaks on false, matching the lvalue overload (currently unreachable for the rvalue path since it is top-level only, but cheap insurance).
• Reversed-copy construction: rargs.resize(...); std::reverse_copy(...) replaced with range-construction std::vector<std::string> rargs(args.rbegin(), args.rend()).
• _process_extras redundant recomputation: the SeparatorOnly branch built the remaining list up to three times; it now computes remaining(false) once. The remaining_size() guard is intentionally kept (it counts non-POSITIONAL_MARK entries only, so it is not equivalent to remaining(false).empty()), preserving exact behavior.
• Dead skipSection variable (Option_inl.hpp): removed the always-false skipSection and its if(!skipSection) guard; the branch now returns unconditionally.
• Dead commented-out code (StringTools_inl.hpp): removed the old escapedChars array experiments.
• remove_quotes(std::string&) duplicated remove_outer: rewritten to delegate to remove_outer, and added the previously-missing declaration of remove_outer to StringTools.hpp.
• Timer GCC 4.7 workarounds (Timer.hpp): removed the _GLIBCXX_USE_NANOSLEEP define (targeted GCC < 4.8, below the documented minimum; the library doesn't call sleep_for) and replaced the AutoTimer constructor reimplementation with using Timer::Timer;. TimerTest passes.
• Dead '\r' checks in the multiline config writer (Config_inl.hpp): the multiline branch is only reachable when is_printable(arg) is true, which excludes '\r' (it is neither isprint nor '\n'/'\t', so it diverts to binary_escape_string). Removed the unreachable || arg.front() == '\r' / || arg.back() == '\r' halves.

Part of #1357