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fix(config): Detect `extends` cycles and enforce a depth cap (#582)

Previously, a configuration file that extended itself — directly or
through a chain of files — would recurse indefinitely, eventually
overflowing the stack. There was no guard against this.

A DFS ancestor stack (`ExtendsStack`) is now threaded through the
`extends` loading helpers. Before recursing into each extended file its
canonicalized path is pushed onto the stack; on return it is popped. If
the same path appears twice on the stack a cycle is detected and
`Error::ExtendsCycle` is returned immediately. A hard depth cap
(`MAX_EXTENDS_DEPTH = 255`) acts as a safety net for the corner case
where path canonicalization fails and lets two logically identical paths
compare unequal; that path yields `Error::ExtendsDepthExceeded`.

Diamond-shaped extension graphs (where two files both extend a shared
third file) are explicitly allowed: a shared dependency only appears on
the ancestor stack while it is actively being traversed, so encountering
it a second time via a different branch does not trigger a false cycle.

Signed-off-by: Jean Mertz <git@jeanmertz.com>

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