Fix BaseUrlTagger.IGNORE_IP_REGEX_START missing f-string prefix

[Chessing234]

Bug

`python/dolma/taggers/url.py:BaseUrlTagger` defines `IGNORE_IP_REGEX_START` as an anchored version of `IGNORE_IP_REGEX`, but the raw-string prefix (`r"..."`) prevents the embedded `{IGNORE_IP_REGEX.pattern}` placeholder from being interpolated:

```python
IGNORE_IP_REGEX = re.compile(r"(127.0.0.1|0.0.0.0|::1)")
IGNORE_IP_REGEX_START = re.compile(r"^{IGNORE_IP_REGEX.pattern}")
URL_REGEX = re.compile(r"(([a-z0-9-_]+.?){2,}|localhost|localdomain)")
ONLY_URL_REGEX = re.compile(f"^{URL_REGEX.pattern}")
ADP_FORMAT_REGEX = re.compile(f"\\|+{URL_REGEX.pattern}\\^")
```

So the compiled pattern becomes the literal six-character run `^{IGNORE_IP_REGEX.pattern}` (the `{...}` is not a valid `re` quantifier, so it's matched as literal characters), which cannot match any IP string.

Root cause

The two sibling anchored regexes immediately below (`ONLY_URL_REGEX`, `ADP_FORMAT_REGEX`) use `f"^...{OTHER.pattern}..."` to splice an existing pattern — `IGNORE_IP_REGEX_START` is written with `r"..."` instead of `f"..."`, so the substitution that was clearly intended never happens.

Because `IGNORE_IP_REGEX_START.match(maybe_ipv6_or_ipv4)` in `parse_line` always returns `None`, the guard

```python
if not self.IGNORE_IP_REGEX_START.match(maybe_ipv6_or_ipv4):
# do not yield the IP if it a localhost
yield maybe_ipv6_or_ipv4
```

always fires, so `127.0.0.1`, `0.0.0.0`, and `::1` lines from hosts-style blocklists are pushed into `self.blocklist` instead of being filtered out as the comment promises.

Why the fix is correct

Switching the prefix from `r` to `f` interpolates `IGNORE_IP_REGEX.pattern` into the anchored regex exactly like the adjacent `ONLY_URL_REGEX` / `ADP_FORMAT_REGEX` patterns do. The resulting regex is `^(127.0.0.1|0.0.0.0|::1)`, which is the same alternation already validated by `IGNORE_IP_REGEX` but anchored to the start — restoring the intended "drop localhost / reserved IPs" behavior in `parse_line`.