feat: package skeleton — DESCRIPTION, NAMESPACE, session/manifest/cache/views

Minimal skeleton for uscogdata v0.1. Internal session layer with lazy
cog_open(), manifest fetch+validate+cache, view registration placeholder.
Depends on DuckDB >=1.0, httr2, jsonlite. inst/schemas/provenance-v1.json
ships the structured provenance JSON Schema.

tests/testthat.R

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+library(testthat)
+library(uscogdata)
+
+test_check("uscogdata")

tests/testthat/helper-fixture.R

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+# tests/testthat/helper-fixture.R
+skip_if_no_corpus <- function() {
+  testthat::skip_if(Sys.getenv("USCOGDATA_FIXTURE_URL", "") == "" &&
+                    !file.exists("~/.cache/R/uscogdata/manifest.json"),
+                    "No fixture corpus available")
+}

tests/testthat/setup.R

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+# tests/testthat/setup.R
+# Point tests at a fixture corpus URL if provided.
+if (Sys.getenv("USCOGDATA_FIXTURE_URL", "") != "") {
+  options(uscogdata.url = Sys.getenv("USCOGDATA_FIXTURE_URL"))
+}

tests/testthat/test-config.R

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+test_that(".cfg resolves defaults, options, and env vars in priority order", {
+  withr::with_options(list(uscogdata.manifest_ttl_secs = NULL), {
+    withr::with_envvar(c(USCOGDATA_MANIFEST_TTL_SECS = NA), {
+      expect_equal(uscogdata:::.cfg("manifest_ttl_secs"), 3600L)
+    })
+  })
+
+  withr::with_options(list(uscogdata.url = "https://opt.example/"), {
+    withr::with_envvar(c(USCOGDATA_URL = NA), {
+      expect_equal(uscogdata:::.cfg("url"), "https://opt.example/")
+    })
+  })
+
+  withr::with_envvar(c(USCOGDATA_URL = "https://env.example/"), {
+    withr::with_options(list(uscogdata.url = "https://opt.example/"), {
+      expect_equal(uscogdata:::.cfg("url"), "https://env.example/")
+    })
+  })
+})
+
+test_that(".resolve_cache_dir falls back to R_user_dir", {
+  withr::with_envvar(c(USCOGDATA_CACHE_DIR = NA), {
+    withr::with_options(list(uscogdata.cache_dir = NULL), {
+      expect_equal(uscogdata:::.resolve_cache_dir(),
+                   tools::R_user_dir("uscogdata", "cache"))
+    })
+  })
+})