Version srcset projection semantics for reproducible receipts #60

The html_attribute_urls projection format needs an explicit parser-semantics version so receipts remain reproducible when srcset parsing changes. A parser correction can change the resulting URL list without changing the projection's declared format.

Minimal synthetic example, containing no measured data:

from lexbench_webfetch.resource_urls import srcset_urls
value = 'https://example.invalid/image?resize=100,200 1x, https://example.invalid/large.png 2x'
legacy = [part.strip().split()[0] for part in value.split(',') if part.strip()]
assert len(legacy) == 3  # Incorrectly treats the URL's internal comma as a separator.
assert len(srcset_urls(value)) == 2

Run through uv run --frozen python. The correct parser should retain commas within URLs. Historical replay should select only explicitly bound semantics, not try alternative parsers until one happens to match a stored result. Keep frozen evidence immutable and reject version mismatches. Add controls for internal commas, descriptors, data URLs and ordinary candidate separators; correct new results through a new versioned revision.

Related report maintenance: PR #59.