frontier --reseal: a record that differs only in built_by is not a change #8571

t27c frontier --reseal writes the old seal back when the new record differs only in sealed_at. It does not do the same for built_by, which changes with every t27c commit. Tick 5 of ledger #8314 produced 17 seal files whose only differences were built_by and sealed_at (PR #8568, closed as churn).

frontier judges the toolchain by output (the gen hashes plus the test runner and zig), never by the producer string (#8095). So a record identical except for built_by is the same verdict.

Acceptance criteria

• --reseal writes the old seal back when only sealed_at and/or built_by differ.
• Hand-written lines do not grow (one line changed).

Boundary

bootstrap/src/service.rs

User Scenarios

• Given a previously sealed frontier record, When t27c frontier --reseal runs and the new record differs only in built_by and/or sealed_at, Then the old seal is written back unchanged.

Requirements

FR-001: The --reseal command MUST treat a record differing only in sealed_at and/or built_by as identical to the previous seal and write the old seal back.
FR-002: The implementation change MUST modify exactly one line in the cited boundary file.

Success Criteria

• t27c frontier --reseal on a record differing only in built_by and sealed_at exits 0 and produces byte-identical output to the prior seal
• git diff bootstrap/src/service.rs shows exactly 1 line changed