fix: deduplicate identical source boundary bars

[pjy008008]

Summary

• deduplicate byte-equivalent canonical bars that appear in overlapping source objects
• keep rejecting same-timestamp bars when any OHLCV value conflicts
• apply the bounded row budget to the unique logical bar set

Why

The production feature backfill found two 1h commands whose 12 authoritative source objects overlap at 147 timestamps. A read-only diagnostic against the exact version-pinned objects confirmed all 147 duplicates for both instruments have identical timestamp and OHLCV values.

Verification

• TDD red: both boundary-overlap tests failed before the implementation
• 68 passed, 2 deselected for the feature output trust-boundary/publication suites with integration tests excluded
• Ruff passed
• mypy passed for 42 source files
• production diagnostic: both affected instruments reported overlap_count=147 and all_identical=true

Local integration note

The two PostgreSQL integration fixtures could not start locally because the checked-out backend central migration bundle is four migrations behind this repository's vendored fixture. CI runs against its own canonical fixture and remains the merge gate.