feat: complete = TRUE fills absent cells with their meaning (#18)

Sparsification (cog_pipeline#64, SB194) stopped the corpus storing the wide
era's explicit zeros, which made absence ambiguous:

  <= FY2011  dense_source   absent => Census published $0
  >= FY2012  sparse_source  absent => not reported, unknown

A wide-era query whose cells were all $0 had begun returning nothing at all,
with no way to get them back -- strictly less than the reader exposed before,
which is why #64 filed this follow-on.

complete = TRUE fills the requested grid from `code_set` and stamps every row
with value_source: "reported", "census_zero" (amt 0), or "not_reported"
(amt NA). The NA is the point. Filling a modern absence with 0 would invent
data, which is exactly the error the representation contract exists to
prevent -- and it makes this strictly MORE informative than the
pre-sparsification corpus, which could not tell a published zero from an
unreported cell either.

Measured on the fixture, Broward County: FY2011 returns 28 reported + 16
census_zero; FY2019 returns 30 reported + 14 not_reported. The five
categories that walkthrough finding F-006 read as "retired at FY2012" now
report themselves correctly as census_zero before and not_reported after.

Scoping decisions, each of which would invent rows if taken loosely:

  - The grid is per government TYPE (code_set.type). Filling against the
    union of all types would give a county cells like "state IG transfer to
    school districts", indistinguishable from real census zeros.
  - NOT is_aggregate, mirroring spending_long/revenue_long. Without it the
    grid offers cells those views never return, so each would fill as a
    phantom $0.
  - Filling happens BEFORE per_capita and inflation, so a census_zero stays
    0 through both and a not_reported stays NA rather than becoming 0.

Two new views (36-representation, 37-code_set) are gated on the manifest
LISTING those tables, not on schema_version. Sparsification did not bump the
version -- the fixture this package shipped against until 2026-07-30 was
already v6 and carried neither table -- so a version gate would register a
view over a missing file and fail at CREATE VIEW time on exactly the corpora
the check exists to tolerate. with_corpus_missing_representation() models
that corpus and asserts the abort.

Refused where the fill would be guesswork, both classed
uscogdata_complete_unsupported: a recipe defines its own component codes and
never touches summary_categories; the intergovernmental leg deliberately
keeps aggregate rows (inst/sql/24-ig_long.sql) so its cells are not the ones
code_set describes.

Expected cell sets in the tests are computed from the corpus parquet
directly, never through the verb -- verifying what a filter does through
that same filter proves nothing.

Closes DoD 2, 3 and 4 of #18. DoD 5 (the cog-api follow-on) is filed
separately.

Suite: 658 pass / 0 fail / 3 skip (was 629/0/3). rcmdcheck clean.

inst/sql/37-code_set.sql

@@ -0,0 +1,3 @@
+CREATE OR REPLACE VIEW code_set AS
+SELECT *
+FROM read_parquet('{url}data/code_set.parquet');