feat: regenerate fixture corpus from Phase P publish tree + committed regen script

Adds data-raw/regenerate_fixture_corpus.R, parameterized by publish-cache
path, so the fixture is never a manual rebuild again. Regenerates the
2019/2020 long partitions (byte-for-byte copy), the full 39,377-row
canonical_fips_xwalk master, the new 117,503-row canonical_alias lookup
table, and summary_categories from the Phase P publish tree; resyncs the
four fixture docs; and hand-builds manifest.json with schema_version 4
and freshly computed sha256/row_count/size_bytes for every shipped file.
Fixture grows from 3.7MB to 5.5MB, well under the 25MB budget.

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