Fix 13 audit findings; per-instance group/subparser copies

[mosquito]

Summary

Two changes on top of 1.10.0: a batch of fixes for latent bugs found by a
full-source audit, and a behavioral improvement that gives every Parser
instance its own copies of declared groups and subparsers.

Audit fixes (13 findings, each verified by a failing test against the pre-fix code)

Metaclass:

• A shared Argument() instance bound to several fields/classes was mutated
  in place — b: str = arg silently required the int inferred for a: int.
  The metaclass now copies before inferring type/nargs/choices.
• Arguments can no longer shadow methods/properties (parse_args,
  print_help, user-defined helpers): previously they were silently replaced
  with ... placeholders, destroying the parser API.
• A bare serve: Serve annotation (no instance) raises instead of silently
  becoming a required CLI argument with type=<Parser class>.
• Subclasses now inherit unannotated arguments, groups, and subparsers
  (previously a subclass lost every tags = Argument(...) / db = DB() /
  serve = Serve() from its base).
• Inherited plain defaults are preserved: name: str = "x" no longer becomes
  a required argument in subclasses.
• Secret(nargs=...) wraps each list element in SecretString (repr no
  longer leaks values).

Config parsers:

• Consistent strict=False: malformed INI/TOML files are skipped like JSON
  instead of raising; strict=True still propagates.
• INIDefaultsParser.BOOL_TRUE_VALUES aliases the shared
  types.TEXT_TRUE_VALUES.
• ConfigAction caches via a None sentinel (an empty config parses once).

Factory:

• EnumArgument(lowercase=True) uses an explicit name map instead of
  str.upper() round-trips, fixing enums with non-uppercase member names;
  enum aliases (WARN → WARNING) remain accepted.

Docs: corrected the false claim that unselected subcommands "retain their
default values" (they raise AttributeError).

Per-instance copies & group reuse

Parser.__init__ now clones the declared member tree (recursively for nested
groups and subparser trees), so:

• Parsing one parser instance never overwrites another instance's group or
  subcommand values; the class-body instances become pristine prototypes.
• Assigning one Group instance to several attributes is now supported —
  each binding is an independent copy. Only cyclic group trees raise
  (at construction time, with a parse-time backstop).

Behavior changes to note

• parser.group is no longer the class-body instance; module-level references
  to a Group/subparser must read values via the parser instance.
• The cycle error fires at Parser() construction instead of parse_args().
• Name collisions with the Parser API now fail fast at class definition
  (such parsers could not work correctly before).

Verification

• 706 tests (incl. 50+ new regression tests), green on Python 3.10–3.14.
• ruff check/format clean; mypy clean (pre-existing tomllib baseline only);
  coverage at the 100% gate.
• Docs blocks executable and passing; Sphinx builds without warnings.