feat: accept corpus schema v6 (FIPS geography harmonization); v6 fixture

Schema v6 (cog_pipeline 2026-07-22) renamed the long table's
fips_state_code/fips_county_code to fips_state_asof/fips_county_asof and added
cog_legacy_state/cog_legacy_county (26 -> 28 cols). This package references
none of those columns and its geography always came from
canonical_fips_xwalk (already present-based), so acceptance is a version-set
bump: supported = c(4L, 5L) -> c(4L, 5L, 6L) in .validate_schema() and
cog_open(). A prominent note in .validate_schema() documents the SILENT
semantic change for raw-long readers: long fips_state/fips_county are now
PRESENT/harmonized geography (carried back per government), not as-of-year.

Fixture regenerated from the published v6 tree (schema_version 6, 28 cols).
Test updates:
  * test-manifest.R: v6 accepted; boundary rejection moves to v7.
  * test-spending.R: the na_rows_excluded pin (0) predated the Task 18 map
    extension, which added E/F/G-prefix discontinued_na rulings (E21/F21/G21,
    Education NEC local, SB184-186). Broward's 2011 partition zero-pads
    exactly those codes: 3 NA-harmonized rows excluded, all amt=0, so the
    excluded AMOUNT pin stays 0. Data-verified against the v6 fixture.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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