proto: stop conformance from downloading real files, ADR the null-clearing gap

1. download.add.json had startMode "now" against a real, large (~6 GB)
   Ubuntu ISO with saveDir hardcoded to /home/sami/Downloads/Programs.
   Against a real veloxd (tests/conformance/run.sh) that's a real
   download into the real user's real home, every single run — it had
   already happened twice. startMode -> "later" (exercises the add path,
   hands nothing to the engine) and saveDir is dropped entirely (resolves
   to saveTo.defaultDir instead, checked against allowedRoots the same
   way). Documented the rule this fixture was breaking in
   contracts/fixtures/README.md so it doesn't happen a third time.

   Auditing the rest for the same shape (now real: capture.offer, D7)
   found a second, subtler instance: capture.offer.take.json's "take"
   admits a real, immediately-started task the same way download.add
   does, and the "Programs" category's saveDir is a migration-seeded
   builtin (~/Downloads/Programs) that no isolated test setup can
   redirect -- so even after pointing the URL at example.org (RFC 2606),
   a real ~6 GB sparse .veloxpart still landed in the real home on the
   declared Content-Length alone. Shrunk to a plausible-but-small 5 MiB.
   Also scoped to "transport": "uds" -- a real "take" persists an active
   task, so replaying the same fixture again on the second live transport
   against the same shared daemon was hitting capture.offer's own
   dedupe-by-URL and failing on a missing taskId, not a bug.
   download.add's other real-URL siblings (errors/*.invalid-path,
   *.invalid-params, *.disk-full) all fail before admission or are
   requires-gated; left alone.

2. ADR 0018: DAEMON can set a nullable field through download.update /
   settings.set but never clear it back to null, because the generated
   C++ parser collapses "absent" and "explicit null" to the same
   std::nullopt for every optional field (contracts/codegen/gen_cpp.py,
   on purpose, and correct for create-style params -- just wrong for
   patch-style ones, which is the only place the schema documents
   "explicit null clears"). Decision: an opt-in x-clearable schema
   annotation makes just those fields std::optional<std::optional<T>> in
   C++ (TS already round-trips this natively); not a blanket rule
   (would retype response fields like TaskSummary.effectiveUrl that have
   no clear-vs-absent distinction to make), not an explicit clear-list
   field (would redesign a wire contract DAEMON already built against
   just to route around a generator gap). Recorded, not implemented here
   -- that's its own PROTO PR (schema annotations + gen_cpp.py + gen_ts.py
   + regeneration + a minor VERSION bump per ADR 0015), not bundled into
   a fixture-safety pass. Left a pointer to the ADR at the generator
   comment it concerns.

3. Re-verified every xfail entry against current deferrals.md rather
   than trust the reasons already on file: D7/D8 (capture.offer/
   getRules), D3d/e/f/g/h/i (rules, queue.reorder, schedule, limiter,
   download.update/refreshUrl) and D9 (settings) have all closed since
   the list was last pruned, so most of it was stale. Removed everything
   that now cleanly passes; kept and re-reasoned everything that doesn't:
   - errors/download.provideAuth.not-found.json stays, as asked: real
     bug, on_download_provideAuth never checks the task exists.
   - category.list.json (mimeTypes -- documented D3a gap), schedule.set.json
     (nextRunAt -- documented D3f gap): unchanged in substance, reason
     text was already accurate.
   - download.probe/get/list/update.json, session.hello.json,
     queue.start/reorder.json, category.remove.json: not bugs -- each
     golden depicts a richer lifecycle/config state (a probed download,
     real queue or category membership, media/grabber capabilities) than
     this harness's fresh, never-started bound tasks and empty isolated
     DB can produce.
   - limiter.get.json: real fixture bug, not a daemon one -- applyToRunning
     is a write-only instruction on limiter.set, on_limiter_get never
     returns it; the golden shouldn't have had it either. Fixed the
     fixture and tools/mockd's own limiter.get, which had the same field
     hardcoded into its in-memory state independent of the fixture file.
   - grabber.*/media.*: still genuinely stub (M4 territory).
   Only remaining unexpected-pass surfaced while re-verifying
   (errors/capture.offer.ignore.json, always "take" instead of "ignore")
   traced to capture.minSizeBytes defaulting to 0 on a fresh daemon,
   making its below-minimum-size scenario unreachable -- not a bug, so
   raised the setting in run.sh's isolated seeding instead of xfailing it.

ctest -L conformance: green, 100% (2/2), ~87s.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01SFeUKLbdHizrJjLBeK7ffz

tests/conformance/run.sh

@@ -160,11 +160,18 @@ if [ -z "$EXTERNAL_UDS" ] && [ -z "$EXTERNAL_WS" ]; then
     for _ in $(seq 1 50); do [ -S "$VUDS" ] && break; sleep 0.2; done
     [ -S "$VUDS" ] || { echo "veloxd did not start:"; cat "$WORK/veloxd.log"; exit 1; }
 
-    # saveTo.allowedRoots defaults to ["~/Downloads"]; download.add.json (fixture) asks
-    # for a saveDir under $HOME/Downloads, so both that and download.add's own isolated
-    # downloads dir need to be allowed roots, or every download.add fixture fails -32011
-    # before the point of this runner is even reached. settings.set is itself a D3 stub,
-    # so this is written straight into the isolated velox.db rather than over the wire.
+    # saveTo.allowedRoots defaults to ["~/Downloads"]; download.add's own isolated
+    # downloads dir needs to be an allowed root too, or every download.add fixture fails
+    # -32011 before the point of this runner is even reached. $HOME/Downloads stays in
+    # the list alongside it: a few fixtures still set an explicit saveDir there
+    # (category.upsert.json, download.update.json) rather than take the default.
+    # capture.minSizeBytes defaults to 0 (nothing is ever "too small"), which makes
+    # errors/capture.offer.ignore.json's below-minimum-size case impossible to reach
+    # against a fresh daemon; raised here so that fixture's scenario is actually
+    # reachable. Written straight into the isolated velox.db, before veloxd has any RPC
+    # session to write it through: settings.set is real now (D9), but this has to be in
+    # place before the very first fixture runs, and setup happens before any connection
+    # exists.
     python3 - "$VXDG/data/velox/velox.db" "$VXDG/downloads" "$HOME/Downloads" <<'PY'
 import json, sqlite3, sys
 db_path, isolated_downloads, home_downloads = sys.argv[1:4]
@@ -174,6 +181,11 @@ db.execute(
     "ON CONFLICT(key) DO UPDATE SET value = excluded.value",
     ("saveTo.allowedRoots", json.dumps([isolated_downloads, home_downloads])),
 )
+db.execute(
+    "INSERT INTO settings(key, value) VALUES(?, ?) "
+    "ON CONFLICT(key) DO UPDATE SET value = excluded.value",
+    ("capture.minSizeBytes", json.dumps(1000000)),
+)
 db.execute(
     "INSERT INTO settings(key, value) VALUES(?, ?) "
     "ON CONFLICT(key) DO UPDATE SET value = excluded.value",