fix(sbd): retain GEAK backend provenance on adoption

[iraj465]

Accepted GEAK optimizations omit backend from their stack entries, so native Session Breakdown places their measured gains in the unattributed backend bucket. Stamp backend="geak" at the GEAK promotion boundary so the recorder preserves engine provenance for both configuration and proven-kernel adoptions. Existing lever and operation kinds continue to identify what changed.

Regression tests exercise real promotion, stack recording and assembled source buckets for backend flags, parameter flags, environment settings, proven kernels and rejected no-gain candidates.

• Linked issue: none; the defect is in Hyperloom’s GEAK integration.
• Validation: 1,041 focused GEAK/V6 tests passed. Full suite: 22,716 passed, 55 skipped, one supervisor restart-limit failure whose diagnostic also occurs on unchanged base 2cc382a; no new failures. Full-tree Ruff, format and REUSE passed. Advisory mypy matches the existing baseline diagnostics after accounting for the inserted source line.
• CHANGELOG.md: entry under [Unreleased].
• Breaking changes: none.
• Single concern: retain engine provenance in accepted GEAK stack entries and native breakdowns.
• Upstream component ticket: not applicable; Hyperloom owns the missing field.