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.

tests/testthat/helper-fixture.R

@@ -85,6 +85,42 @@ with_doctored_schema_version <- function(version, code) {
   force(code)
 }
 
+# Copy the bundled fixture to a temp dir with representation.parquet and
+# code_set.parquet removed (and dropped from the manifest's metadata list),
+# then run `code` against it. Models a corpus published BEFORE sparsification:
+# schema_version is left alone deliberately, because it was never bumped for
+# that change -- the pre-sparsification fixture this package shipped until
+# 2026-07-30 was schema v6 and carried neither table. Presence in the manifest
+# is therefore the only honest signal, and this helper is what proves the
+# package keys off it rather than off the version number.
+with_corpus_missing_representation <- function(code) {
+  src <- fixture_corpus_path()
+  tmp <- withr::local_tempdir(.local_envir = parent.frame())
+  file.copy(list.files(src, full.names = TRUE), tmp, recursive = TRUE)
+
+  dropped <- c("representation.parquet", "code_set.parquet")
+  file.remove(file.path(tmp, "data", dropped))
+
+  manifest_path <- file.path(tmp, "manifest.json")
+  m <- jsonlite::fromJSON(manifest_path, simplifyVector = FALSE)
+  m$files$metadata <- Filter(
+    function(f) !basename(f$path) %in% dropped, m$files$metadata
+  )
+  writeLines(
+    jsonlite::toJSON(m, auto_unbox = TRUE, pretty = TRUE, null = "null"),
+    manifest_path
+  )
+
+  old_url <- Sys.getenv("USCOGDATA_URL", unset = NA)
+  uscogdata:::cog_close()
+  Sys.setenv(USCOGDATA_URL = paste0(tmp, "/"))
+  on.exit({
+    uscogdata:::cog_close()
+    if (is.na(old_url)) Sys.unsetenv("USCOGDATA_URL") else Sys.setenv(USCOGDATA_URL = old_url)
+  }, add = TRUE)
+  force(code)
+}
+
 # Copy the bundled fixture to a temp dir with summary_categories.parquet
 # rewritten to drop every M/L (intergovernmental) row, then run `code`
 # against it with a clean session (mirrors with_fixture_corpus()/