Merge cascading/hierarchical configuration files

[kdeldycke]

This depends on #651 to implement the configuration file hierarchy traversal.

This is about merging configuration files at different position in the file system hierarchy.

See:

• https://eslint.org/docs/latest/use/configure/configuration-files#cascading-and-hierarchy
• https://beta.ruff.rs/docs/configuration/#pyprojecttoml-discovery

• click-extra/click_extra/config.py

  Lines 119 to 120 in 6af02e8

  	- ``-C``/``--config`` option, which cannot be used to recursively load another
  	configuration file.

Use cases: mono-repos (and reused workflows?).

[kdeldycke]

merge_default_map() now uses collections.ChainMap instead of dict.update() to layer config values into ctx.default_map (see the TODO that was at config.py:976-977).

This is a prerequisite for cascading config file merging. With the old dict.update(), all config sources were flattened into a single dict, losing their origin. With ChainMap:

• Each call to merge_default_map() adds a new layer rather than merging destructively. When multiple config files are loaded (e.g. ~/.config/app.toml then ./app.toml), the local file's layer sits in front and wins on key lookup automatically.
• ctx.default_map.maps exposes the individual layers, so --show-params could report which config file provided each value.
• Click compatibility is preserved — Click only calls .get() on default_map (in Context.lookup_default()), which ChainMap supports with the correct first-match-wins semantics.

The remaining work for this issue is the config file hierarchy traversal (discovering and loading multiple files), which was tracked in #651. Once that lands, the ChainMap layering is ready to receive each file as a separate layer with explicit precedence.