feat(examples): dual-emit v1+v2 formats on reference seller (#741 polish)

[bokelley]

Summary

Closes the conformance gap between the canonical-formats projection layer (#741) and the AdCP storyboard runner's reference seller. Half 1 + half 2 of #741 shipped the projection layer; this PR makes examples/seller_agent.py actually exercise it.

What changed

Every product in examples/seller_agent.py's PRODUCTS catalog now carries BOTH wire shapes:

• v1 — format_ids: [{agent_url, id}] (unchanged).
• v2 — format_options: [ProductFormatDeclaration] with format_kind="image", sized params (width/height + asset_source + image_formats), and v1_format_ref pointing back at the v1 format_id.

A new _image_format_options(...) helper builds the v2 declaration; the docstring documents the dual-emit pattern as the template adopters should follow during the 3.0 → 3.1 migration window.

Why this matters

• 3.0 buyers continue reading format_ids[] from the wire, unchanged.
• 3.1 buyers now see real format_options[] declarations — the storyboard runner exercises that path end-to-end against the reference seller for the first time.
• SDK projection round-trips. project_product_to_v1 on each product emits refs that round-trip back to the source declaration via find_declaration_by_v1_format_id.

Tests

• New tests/test_seller_agent_dual_emit.py pins 3 invariants over the 6 products:
  1. Every product dual-emits.
  2. Every v2 declaration's v1_format_ref matches the product's v1 format_ids.
  3. SDK projection rebuilds the v1 emit from the v2 surface with zero advisories.
• 5258 tests pass locally.
• Both existing storyboard runs still pass — the v2 surface is purely additive on the wire.

Polish PR backlog (remaining)

Tracked as separate PRs after this one:

• B: rename projection.py → v2_to_v1.py for symmetry
• C: typed Divergence discriminated union
• D: adcp.canonical_formats.fixtures public module
• E: group_declarations_by_product helper
• F: _versions_overlap forward-compat (Argus feat(canonical-formats): public API + v2→v1 projection (#741, half 1 of 2) #841 deferred)

Refs: #741

[aao-ipr-bot]

LGTM. The right shape for the 3.0 → 3.1 migration window: v1 format_ids[] stays load-bearing on the wire, v2 format_options[] is purely additive, and v1_format_ref is what makes the SDK's projection round-trip.

Things I checked

• Dict literal produced by _image_format_options (examples/seller_agent.py:172-208) validates against ProductFormatDeclaration — format_kind="image" is in CanonicalFormatKind (canonical_format_kind.py:11), params keys (sizes, asset_source, ssl_required, image_formats) miss every credential-shaped suffix the _reject_credential_shaped_extras gate (canonical_decl.py:238) screens for. No fail-closed surprise at validation time.
• All 6 products dual-emit with a v1↔v2 anchor: each format_options[0].v1_format_ref[0] matches the product's format_ids[0]. The three test invariants (tests/test_seller_agent_dual_emit.py:55, 64, 81) pin exactly that, plus zero advisories from the projection.
• products_response(PRODUCTS, cache_scope="public") at examples/seller_agent.py:502 and :512 is the wire sink — the new field flows through the standard serializer, no bespoke path.
• Test imports the seller via importlib.util.spec_from_file_location and cleans up sys.modules in finally (test file:46-48). Hermetic; doesn't leak the seller_agent name into other tests.
• Happy path unchanged for 3.0 buyers — format_options is additive, no existing field touched.

Minor nits (non-blocking)

1. Import reshuffle hitched to the feature commit. INSECURE_ALLOW_ALL moved into the main adcp.server import block (examples/seller_agent.py:23) — pure ordering, unrelated to dual-emit. Could have lived in its own commit, but trivial enough not to matter.
2. _image_format_options is single-size by construction. Signature takes width, height and emits one sizes[] entry. Fine for the reference catalog, but the name suggests broader applicability — adopters copying it as a template will need to widen it the first time they declare a multi-size declaration. Worth either renaming (_single_size_image_format_options) or accepting a sizes iterable when this gets lifted out of examples/ into adcp.canonical_formats.fixtures (item D on the deferred backlog).

Approving.