fix(cpp): reject UTF-8 name collisions before CLI output

[ColinLeeo]

UTF-8 replacement can make distinct TsFile identifiers render as the same name. The CLI previously rejected only some NDJSON header collisions, returned exit code 3 for row queries, and could miss collisions in metadata, CSV/table output, or empty query results. Multi-object export could also create its destination before detecting a later object's collision.

This change makes name collisions consistently return exit code 2 before writing stdout or creating export files/directories:

• Share normalized-name validation across output formats and preflight names in ls, schema, count, stats, head, cat, and export.
• Validate the selected schema before time/row filters, including empty results, and check merged TAG headers in table statistics.
• Validate every selected export object before opening output files or creating the destination directory, preserving existing targets with --force.
• Keep valid projections and independent column scopes working, including repeated metadata names belonging to the same original object.

Follow-up for #997.

Validation:

• All 225 C++ CLI tests in 22 suites passed, including the new UTF-8 collision regression matrix and existing golden-output tests.
• 90 standalone CLI fixture probes passed (exit code 2 and empty stdout).
• ./mvnw -o spotless:check -P with-cpp passed.
• git diff --check passed.