chore(types): pin AdCP spec to 3.1.0-rc.6

[bokelley]

Pins the bundled AdCP spec version to 3.1.0-rc.6 and adds the matching schema cache generated from the published bundle.

Regenerates Pydantic types and exports so the rc.6 event tracking surface is available, including EventSurface, event surface/action_source fields, content progress fields, and follow/content_view/watch_milestone event types.

Updates SCHEMA_DELTAS.md to summarize the generated field changes.

Validated with make regenerate-schemas, pre-commit hooks during commit, and focused pytest schema/version/public API suites.

[aao-ipr-bot]

LGTM. Clean spec pin from 3.1.0-rc.4 to 3.1.0-rc.6 — additive enum values, optional new fields, no shape changes on the wire.

Things I checked

• src/adcp/ADCP_VERSION (3.1.0-rc.4 → 3.1.0-rc.6) matches the new schemas/cache/3.1.0-rc.6/ directory.
• SCHEMA_DELTAS.md accurately enumerates the diff — EventType +{content_view,follow,watch_milestone}, new EventSurface class, Event.surface, EventCustomData.{progress_percent,progress_seconds}, EventSource.{action_source,surface}.
• Every new field on existing models is | None (verified core/event.py:46-49, core/event_custom_data.py:60-75). EventSurface.category is the one required field, and EventSurface itself is always optional. Non-breaking — chore(types): is the right prefix, no ! needed.
• Codegen timestamps (2026-06-01T00:32:59+00:00) and # generated by datamodel-codegen headers consistent across regenerated files — not hand-edited.
• Only three hand-touched files outside generated_poc/ and schemas/cache/: SCHEMA_DELTAS.md, ADCP_VERSION, _generated.py. aliases.py, _ergonomic.py, _forward_compat.py untouched, as expected for an additive regen.
• _generated.py Category re-pointing (governance.sync_plans_response.Category → core.event_surface.Category) is internal-only churn — Category is not exported from src/adcp/types/__init__.py.__all__, and grep finds no non-allowlisted importer of Category from _generated. The only re-point in the diff.
• code-reviewer: clean, one follow-up.

Follow-ups (non-blocking — file as an issue)

• EventSurface is in _generated.__all__ but not in src/adcp/types/__init__.py.__all__. Buyers will see Event.surface: EventSurface | None on the public model but cannot from adcp.types import EventSurface to construct one. Same gap for the new Category enum in core/event_surface.py:14-25 if adopters want to set surface.category from a typed value rather than a string. Codegen doesn't edit __init__.py, which is why this slipped — add to the post-regen fix script or to the public re-export block.

The chore-vs-feat call is interesting given that adopters do get new surface fields, but the regeneration is mechanical and the prefix matches how prior rc.X pins have shipped.

LGTM. Follow-ups noted below.