Build the v2 writer before the reader that consumes it

[micahflee]

Reorders the #91 epic so Cyd Bluesky archive v2 export lands before import. No product code changes — three ADRs, plus issue edits already applied to #91, #96, and #98.

Why

Reader-first ordering left every reader issue with no way to manufacture test data. The writer is the only component that can turn Bluesky saved data into a Cyd Bluesky archive, so building it last meant #96 and #97 would be developed against synthetic fixtures alone.

Nothing required that order any more. ADR 0004 gated writer work on #90; #90 closed on 2026-07-31. The epic itself said "the writer and complete v2 conformance are blocked by #90" and nobody moved #98 afterward, so it was still carrying a lapsed dependency.

The surviving concern is a release concern, not a construction one: people should not get an export button before readers are proven. #100 already covers exactly that, unchanged.

Changes

File	Change
0004	Renamed and rewritten around the built-vs-enabled split. Building and testing the writer is ungated; enabling it for users waits on #100.
0008	One sentence placing Mobile-generated fixtures under the pinned bundle rather than beside it.
0016	New. Records why the writer seeds fixtures from a curated test account, and why that does not make them contract authority.

Nothing in the repo referenced 0004's old filename.

Dependency graph

before:  93 ─→ 95 ─→ 96 ─→ 97 ─┐
         90 ──────────→ 98 ────┴─→ 99 ─→ 100
                        ↑______ blocked by 96

after:   93 ─→ 95 ─┐
         90 ───────┴─→ 98 (+fixtures) ─→ 96 ─→ 97 ─→ 99 ─→ 100

#97, #99, and #100 needed no edits.

Worth a second look

• 0004 reverses a recorded decision rather than adding to one. The repo has no supersede/status convention in docs/adr/, so it is rewritten in place under its original number.
• The writer's output is unvalidated by any reader until Restore a complete Bluesky local account for offline browsing #96 lands. The cheap answer is a conformance checker in scripts/archive-contract/ — test_bundle.py already does manifest reconciliation and digest verification, so pointing that machinery at an arbitrary .cyd against the pinned schema.sql is mostly lifting existing code. Once Restore a complete Bluesky local account for offline browsing #96 lands, export → import → export round-trip equality becomes available, which is a stronger test than either half has today.
• The epic edits went slightly beyond the four issue changes originally agreed. Implement Cyd Bluesky archive v2 and cross-client portability #91 carried five stale references to the Preserve full Bluesky images and videos for offline browsing #90 gate across Solution, Constraints, Out of Scope, and Further Notes. Sentences about release order — user story 33, the release-gating matrix — were deliberately left alone.
• Fixture size is now an acceptance criterion on Create a point-in-time Cyd Bluesky archive v2 export #98. data.db for a curated account is tens of KB; media is the whole story. Curating the test account before export matters more than trimming afterward, since committed media persists in repository history.

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