Dictionary lookup collides for distinct column paths with the same dot string

[costas-db]

Problem

DictionaryPageReader keys column metadata and cached dictionary pages by ColumnPath.toDotString(). These distinct component paths therefore collide:

Top-level field named `a.b`: ["a.b"]
Nested field `b` in `a`:     ["a", "b"]

The later column overwrites the earlier entry under the shared "a.b" key.

Reproduction

A generic test uses two dictionary-encoded string columns with disjoint values:

["a.b"]    -> ["one", "two"]
["a", "b"] -> ["nested-one", "nested-two"]

Requesting the top-level dictionary currently returns:

expected: ["one", "two"]
actual:   ["nested-one", "nested-two"]

Proposed direction

Key the metadata and dictionary cache maps by component-based ColumnPath rather than flattened strings. This is an in-memory identity fix with no on-disk format change.