test(e2e): assert exact reporter event-type sequence per example

[bketelsen]

Summary

tests/e2e/examples_test.go's JSON-mode check only verified event shape
(every type known, every timestamp RFC3339) and that the stream ended on
complete. A missing, duplicated, or reordered intermediate Step,
Progress, Message, Warning, or Error event from any of the four
runnable examples would pass unnoticed as long as the final event was still
complete. Adds an expectedEventSequence helper that derives the exact
JSON EventType sequence each example must emit from its fixed fixture data
in _examples/<name>/main.go, and asserts the decoded sequence matches it
exactly. Updates tests/e2e/README.md to document the stronger runtime
assertion. No reporter behavior or example fixture changed — test and docs
only.

Checks

• make check — gofmt leaves no diff, golangci-lint run
  (.golangci.yml) clean, go test -v ./... green (unit + tests/e2e)
  ok github.com/frostyard/std/reporter 0.003s ok github.com/frostyard/std/tests/e2e 5.045s coverage: 74.6% check-coverage: observed total statement coverage 100.0% (required >= 95.0%) check-coverage: OK
• go test -race -short ./... green
  ok github.com/frostyard/std/reporter 1.012s ok github.com/frostyard/std/tests/e2e 6.222s
• go.mod still declares no dependencies (stdlib only) — unchanged, no
  import added outside stdlib and github.com/frostyard/std/reporter
• New or changed behavior has focused tests, including failure paths;
  exact output changes update docs/specs/reporter-package.md — this PR
  only strengthens an existing test's assertion; no reporter output
  behavior changed, so docs/specs/reporter-package.md is untouched.
  Verified the new assertion actually catches a regression by
  temporarily deleting one Warning call from _examples/deploy/main.go
  and re-running go test -run TestExamples/deploy/json -v ./tests/e2e/...:
  it failed with
  event type sequence = [... want step message warning ...], then
  restored the file before committing.

Docs housekeeping

• AGENTS.md, docs/design/overview.md, docs/specs/* updated for
  behavior or convention changes — not applicable; tests/e2e/README.md
  (the e2e suite's own doc) is updated instead, since no AGENTS.md or
  docs/specs/* claim changed.

Protected boundaries

Not applicable — this PR touches only tests/e2e/examples_test.go and
tests/e2e/README.md; no .github/workflows/**, .goreleaser.yaml,
.svu.yaml, or policies/agent-governance.json files are touched.

Risk classification

Low — test-only strengthening of an existing e2e assertion plus a
matching doc update; no production code, reporter behavior, example
fixtures, or CI/release configuration changed. make check, race tests, and
the docs integrity check all pass unchanged.

Verification

• node scripts/check-docs.mjs green
  ok docs_index_coverage: 1.000 (required 1) ok link_integrity: 1.000 (required 1) ok symlink_resolution: 1.000 (required 1) checked: 12 docs, 153 links, 9 symlinks
• Checked against the
  PR review rubric