Add config_argument: user-supplied config file as argument defaults

[mosquito]

Summary

config_files= lets the developer preset defaults from files chosen at
construction time. The new Parser(config_argument="--config") adds a CLI
flag so the end user can point at a config file whose values become
argument defaults, resolved at invocation time:

class CLI(argclass.Parser):
    host: str = "localhost"
    port: int = 8080

parser = CLI(config_argument="--config")          # or ("-c", "--config", ...)
parser.parse_args(["--config", "prod.ini", "--port", "1234"])
# host comes from prod.ini, port from the CLI

Priority chain extends naturally:

declared defaults < config_files < config_argument file < env vars < CLI args

Implementation

Two-pass parsing (the classic argparse chicken-and-egg: defaults must be known
before parsing, but the config path arrives as an argument):

1. A throwaway pre-scan parser — it neither prints nor exits — extracts only
   the config flag value from argv (syntax errors are left for the real parser
   to report with proper usage text).
2. The file is loaded through the shared config_parser_class machinery
   (INI/JSON/TOML or a custom AbstractDefaultsParser) and layered over the
   constructor config defaults.
3. The real parser is built as usual — required-relaxation, type-aware
   conversion, group/nested-group sections, and --help defaults all come
   from the existing defaults pipeline.

Details:

• An explicitly passed path that is missing or unparsable raises
  ConfigurationError — unlike config_files, which stays a lenient search
  list.
• The runtime defaults layer lives only for the duration of one
  parse_args() call (no staleness between parses).
• New Parser.loaded_config_files property reports applied files in priority
  order.
• No cascade into subparsers (consistent with config_files).

Also fixes a related pre-existing bug in ConfigAction (argclass.Config()):
an explicit --config file was merged before search_paths, so preset
files silently overrode the user's explicit choice. The explicit file now
wins.

Tests & docs

• 19 new tests (tests/test_config_argument.py): the priority chain
  pair-by-pair, required-satisfied-by-config, nested group sections, JSON
  parser class, aliases/=-form, help output, error cases, re-parse
  isolation, and the ConfigAction ordering fix.
• Docs: new section in config-files.md (executable example), README example,
  api.md cross-references, Parser.__init__ docstring (flows into the API
  reference via autodoc), feature bullet in index.md, pitfalls table row.

Verification

• 725 tests green on Python 3.10–3.14; doc blocks executable and passing.
• ruff check/format clean; mypy clean (pre-existing tomllib baseline only);
  coverage at the 100% gate; Sphinx builds without warnings.