Port the pkg/fidelity generic differ from archive/fidelity-batch-2026-07-25

[thesprockee]

The 2026-07-25 fidelity effort is preserved as the pushed tag archive/fidelity-batch-2026-07-25 (tip 9b2471d, 6 commits, ~4545 insertions). It was not merged during the 2026-08-26 branch cleanup. Recovering it: git checkout -b <name> archive/fidelity-batch-2026-07-25.

What it contains

• pkg/fidelity/{differ,keyscan,verdict}.go plus tests — a generic field-walking differ, roughly 2400 lines
• pkg/conversion/fidelity.go, fidelity_verify_test.go, fidelity_bench_test.go
• Four new audit tests: roundtrip_baseline_audit, roundtrip_baseline_keyset, roundtrip_v2_findings, roundtrip_baseline_audit_status
• A rewritten cmd/tapedeck/verify.go

Why it did not merge

1. Stale module path. 35 files import github.com/echotools/tape/pkg/.... The module is now github.com/echotools/tape/v4. The branch does not compile against main.
2. It is an architectural swap, not a merge. It deletes ~610 lines of currently-green hand-rolled comparators in pkg/conversion/roundtrip_v2_test.go and routes that coverage through pkg/fidelity instead. Taking the branch side drops coverage that passes today; taking main's side discards the point of the branch.
3. Mechanically the conflict surface is small — only BUGS.md (deleted on main, bugs now live here as issues) and roundtrip_v2_test.go. The remaining ~4300 lines are new files.

Why it is still worth doing

A generic differ that walks every field beats per-field comparators that have to be remembered and hand-extended — the recurring defect in this repo is a complete write path against a partial read path, and a field-walking differ catches exactly that class. The port is tractable; it just needs a dedicated pass with the suite green at the end, not a merge.

[thesprockee]

Before porting the ~2400-line pkg/fidelity: the generic differ can probably be built on protocmp.Transform() + cmp.Diff. go-cmp v0.7.0 is already in go.sum, so no new dependency.

[thesprockee]

Ported in b37762d (parallelized in 8fc42f5). pkg/fidelity is on /v4 and is the verdict of tapedeck verify <file.echoreplay>; the hand-written comparator in roundtrip_v2_test.go is replaced by the same verification (TestRoundTripBAC, TestRoundTripBACAudit). Adjusted to the realistic-archiving bar in docs/format-design.md §4 — tests: TestClockDisplayToleratesOneCentisecondOnly, TestScannerSeparatesZeroValuedUnknownKeys, TestAbsentOnBothSidesIsReportedNotFailed, TestEmptyTeamsPairByName, TestEmptyBonesMessageIsAbsent, TestVerifyFailsOnFabricatedLastThrow, TestVerifyReadsUnzippedRecordings. Measured on 14 real recordings: 11 pass, 3 fail only on #59.