CLI: surface result.debug via --json field or stderr log

[kaznak]

Problem

Defuddle({ debug: true }) populates result.debug = { contentSelector, removals: DebugRemoval[] } so callers can see which step dropped what. The CLI accepts a --debug flag and passes debug: true through, but nothing in the CLI ever surfaces the resulting result.debug:

• the --json payload's field list in src/cli.ts does not include debug, so it is dropped silently.
• nothing is written to stderr either.

The practical effect is that --debug is a no-op for CLI users. The most common reason to enable it on the command line — figuring out which removal pass stripped an element while developing a site-specific extractor — therefore can't be served by the CLI today.

Proposal

Route result.debug through the CLI:

1. With --json --debug — include the existing DebugInfo payload in the JSON object, alongside the other fields:

{
  "content": "...",
  "title": "...",
  ...,
  "debug": {
    "contentSelector": "html > body > main > article",
    "removals": [
      { "step": "removeByContentPattern", "reason": "blog metadata list", "text": "Hugging Face ModelScope" },
      ...
    ]
  }
}

Field shape is unchanged — it is DebugInfo from src/types.ts. Machine consumers (jq pipelines, extractor-development scripts) can read it directly.

2. With --debug and no --json — format the same payload as a one-line-per-removal human-readable log and write it to stderr. Stdout still gets the article content unchanged, so existing pipelines keep working. Example stderr output:

# defuddle --debug
contentSelector: html > body > main > article
removals: 12
[removeBySelector] (.ad) — Advertisement
[removeByContentPattern] blog metadata list — Hugging Face ModelScope
[removeByContentPattern] breadcrumb navigation list — Home > AI > World Models
...

3. Without --debug — no behavioral change. JSON output omits the debug field, plain output is identical to before.

Implementation lives entirely in src/cli.ts: a new formatDebugLog() helper, a debugLog?: string field on the parseSource return value, and the action wrapper writes it to process.stderr.

Tests

tests/cli.test.ts adds:

• --debug --json embeds debug field with contentSelector (string) and removals (array). debugLog is undefined (the JSON path owns the output).
• --debug without --json returns a debugLog string starting with # defuddle --debug and containing contentSelector: and removals:. The primary output field stays content-only.
• Without --debug: JSON has no debug field, plain mode has no debugLog.

All 371 existing tests continue to pass.

Notes

• The internal DebugInfo / DebugRemoval types are imported as type so the CLI bundle does not pull in any runtime code from src/types.ts.
• The stderr formatter trims whitespace from r.text to keep each removal on one line; the API's text field already gives a short preview, so no extra truncation is done.
• Independent of --source-url (CLI: add --source-url flag for site-specific extraction on stdin/file input #322) and --extractor (CLI: add --extractor flag and defuddle/extractor public entry #328). All three combine naturally: --extractor my.mjs --debug --json is the workflow this PR is aimed at.