Remove legacy fixture migration paths and simplify resource normalization

[AlexanderOnischenko]

Motivation

• Simplify fixture migration by removing support for legacy resource layout heuristics and require canonical resources/<stem>/<resource_id>/... paths in bundles.
• Make migration behavior deterministic: only normalize path syntax (backslashes) and validate resource files exist instead of guessing/moving legacy locations.

Description

• Change fixture_migrate in src/fetchgraph/tracer/fixture_tools.py to reject non-canonical resource paths and to only normalize data_ref.file to the canonical POSIX path under resources/<stem>/<resource_id>/... instead of handling multiple legacy patterns or moving files.
• Remove file-content collision checks and actual resource-moving logic from migration, and drop the now-unused filecmp import.
• Update tests in tests/test_tracer_fixture_tools.py to remove legacy scenarios and assert that canonical paths are left unchanged or normalized (including backslash normalization) and that no files are moved when paths are already canonical.
• Modified files: src/fetchgraph/tracer/fixture_tools.py and tests/test_tracer_fixture_tools.py.

Testing

• No automated test suite was executed as part of this change (no CI run requested).

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Codex Task