fixtures: commit v1 CBOR fixtures under tests/fixtures/v1 (required by T0.3 spec)

The gen_fixtures regen tool was written but never executed, so no Cbor
fixture bytes actually shipped in commit a1a3737. Without them the harness-
level contract in T0.3's Acceptance is false — 'A serialized v1 fixture
checked into the repo' fails silently: check_fixtures_present panics with
'need at least one v1 fixture loaded from disk', and variant coverage is
silently empty, which the harness wrote as a known False pass ('Coverage
drift... the directory walk still passes because it only checks what is
there').

Added files: 8 .cbor records (one per WorkEvent variant), all at
SchemaVersion(1). The existing _regen_fixtures #[test] gated on FIXTURE_REGEN=1
remains valid — but in production the Verify command runs without that env var,
against committed bytes. That is exactly what makes it not a False pass.

Also widen .gitignore was blocking 'tests/fixtures/' globally; per T0.3 spec
the harness fixture path is data shipped with the repo and must be tracked.

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