fix(corpus): report unrecognised statistics rows

[Boulea7]

When statistics rows stop matching the supported <tr id="..."> shape, the reader currently reports ErrNoFigures and Accounts skips the HTML account. Return a report-reader error when either statistics table contains unrecognised data rows, while retaining ErrNoFigures for genuinely empty tables such as the recorded Gatling 3.13.1 report.

The regression changes every data-row opening, including ROOT, in a temporary copy of the recorded 3.15.1 report and checks both error classification and Accounts propagation. It also covers table boundaries, headings and direct table rows. The change uses the existing table boundaries and standard-library regular expressions.

Closes #105. Specification: 012-report-reader-row-shape.

Validation with Go 1.27.2: focused regressions, core race/shuffle tests, recorded-corpus integration (349 cases), build, vet, module checks, workflow pins and six shell suites passed. Coverage floors passed at 93.6% overall (84.0% for internal/corpus).

Local lint/format checks with golangci-lint 2.14.0 remain nonzero: the same five lint findings and four formatting hunks reproduce on the unchanged base, with no additional findings from this change. The configured Go 1.26.8 download timed out; that toolchain, minimum Go 1.25 and CI's lint 2.12.2 were not verified locally. Hosted CI still needs to cover the pinned vulnerability, NFR and API-compatibility checks, fresh Gatling canary, separate peak-memory test and PR fuzz budget before merge.

[Boulea7]

The CI run is awaiting approval (action_required, with no jobs executed). Could a maintainer approve it so the repository checks can run?

Please also assign this PR to v0.2.0 The gates hold, matching #105, before merge as required by the project's contribution workflow.