feat(buffers): SUT-side antithesis underflow assertions

[blt]

Summary

Adds the antithesis_sdk + antithesis-instrumentation dependencies to
vector-buffers and three #[inline] assert_always! guards at the unchecked-
arithmetic sites that drive the #21683 accounting-underflow cluster:
Ledger::decrement_total_buffer_size, Ledger::get_total_records, and the
reader.rs:524 data-file size-delta. The assertions are no-ops outside the
Antithesis environment and #[inline] so they fold away in production builds.

Stack (Antithesis-driven disk buffer v2 data-loss investigation):

• docs(buffers): disk_v2 data-loss research scratchbook #25524 — research scratchbook (docs)
• chore(unit tests): failing disk_v2 bug demonstrations #25525 — failing bug-demonstration tests
• feat(buffers): SUT-side antithesis underflow assertions #25526 — SUT-side assertions
• chore(tests): disk_v2 antithesis test harness + exercisers #25527 — test harness + exercisers

How did you test this PR?

cargo build -p vector-buffers / cargo nextest run -p vector-buffers. Under
Antithesis, the decrement_total_buffer_size guard already fired organically
(reproducing #21683). make build-licenses regenerated the 3rd-party license
inventory for the new deps.

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Related: buffer_byte_size has value 18446744073709548000 #21683

[blt]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• chore(tests): disk_v2 antithesis test harness + exercisers #25527
• feat(buffers): SUT-side antithesis underflow assertions #25526 👈 (View in Graphite)
• chore(unit tests): failing disk_v2 bug demonstrations #25525
• docs(buffers): disk_v2 data-loss research scratchbook #25524
• master

This stack of pull requests is managed by Graphite. Learn more about stacking.

[datadog-vectordotdev]

✨ Fix all issues with Cursor

⚠️ Warnings

🧪 7 Tests failed

buffer_usage_data::tests::drop_newest_drops_should_increment_component_discarded_metric_issue_24606 from vector-buffers     (Fix with Cursor)

Test has failed

variants::disk_v2::tests::acknowledgements::rejected_delivery_should_not_advance_acks_finalizer_status_discard from vector-buffers     (Fix with Cursor)

Test has failed

variants::disk_v2::tests::invariants::delete_completed_data_file_size_delta_should_saturate_reader_524 from vector-buffers     (Fix with Cursor)

Test has failed

View all

ℹ️ Info

No other issues found (see more)

❄️ No new flaky tests detected

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