feat: emit per-check progress to stderr during runs

[danielcalvolopez]

Closes #105

What changed

Between the Running checks on <site>... banner and the final report, afdocs emitted nothing, so on a slow or hostile site there was no way to distinguish "working through many slow requests" from "hung" without inspecting the process from outside. This adds lightweight check-level progress on stderr:

Running checks on docs.alkimi.org...
[1/23] llms-txt-exists... done (259ms)
...
[7/23] llms-txt-directive-md... done (33 tested, 2 fetch errors, 8s)
...
[23/23] auth-alternative-access... skipped

• Runner: RunnerOptions gains an optional onProgress callback that fires a start and complete event per selected check, carrying index/total, the CheckResult, and wall-clock duration. The library API stays output-agnostic; event types are exported from the public API.
• CLI: renders events as numbered stderr lines. The start fragment is written without a newline, so a stalled check leaves its name visible on screen — you can spot a hang within one check's duration. Completion lines include the tested-page count and the fetch-error count from the check's existing details, doubling as an early live signal of the bot-protection interference pattern from Run-level diagnostic for bot-protection interference (spec v0.6.0 bot-protection-interference) #104.
• -q, --quiet: suppresses progress and the banner for clean CI logs.
• Progress goes to stderr for every format, so parseable stdout (json, piped scorecard) is never contaminated.

Per the issue's notes, this implements check-level granularity only (the best effort/value point); per-page ticks within multi-page checks are left as a possible follow-up.

Testing

• 15 new tests: runner event emission (ordering, numbering, skip/dependency-skip/error paths), progress-line formatting, and CLI wiring (--quiet, json stdout stays parseable). Full suite: 1360 passing.
• Verified live against real sites: default run shows progress, --quiet produces zero stderr bytes, -f json > report.json stays parseable while progress streams to the terminal.
• Docs updated in docs/reference/cli.md.