Preserve directory semantics when normalizing terminal dot segments

[rupayon123]

Preserve directory semantics when Path.normalize() removes a terminal /. or /... For example, /a/b/c/d/.. now normalizes to /a/b/c/, so a later relative join resolves within that directory instead of replacing its last component. Relative paths, root normalization and idempotence are covered too.

This addresses the normalization part of #159. It deliberately does not change raw join() behavior: URI resolution merges a relative reference with the base before removing dot segments. Reference: RFC3986 section5.2.

Validation on Python3.11.15:

• Six failing regression subcases before the patch; both new test methods pass afterward.
• Full nose2 run:79 tests, with the same one failure and two errors as unchanged master's77 tests. These are existing test_odd_urls, test_hosts and test_netloc failures; related fixes already have open PRs (Fix double '//' on URLs with empty authority and '//'-prefixed path (#185) #197/Fix: tests/test_furl.py::test_hosts (and the analogous case in test_netloc)... #198).
• Flake8 passes for the changed source and test file; git diff --check passes.

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