ERMAIN-387: create-appointment confirm path (outlook.create_appointment + erato-appointment fence)

[bdart]

Implements ERMAIN-387 on top of the merged #801 seam: the model's picked slot becomes a user-confirmed, prefilled displayNewAppointmentForm (Saved/Sent manually — no Graph/EWS write anywhere).

Flow: outlook_schedule step 4 now emits one erato-appointment fenced JSON block + one propose_client_action("outlook.create_appointment") + a prose restate. The add-in renders the fence as an appointment summary card; the (always-ask enforced) confirm card shows time + attendees and opens the prefilled form.

Pieces:

• outlook.create_appointment in IMPLEMENTED_CLIENT_ACTIONS, partitioned by KIND (email vs appointment) so the email renderer can never surface an appointment action as a reply button (and vice versa).
• New erato-appointment fence dispatch in the shared library via registry key EratoAppointmentCodeBlock (default: plain code block; web unaffected).
• OutlookArtifact.bodyFormat is now optional and gates everything email-shaped (drifted-tag rescue, whole-body card) — scheduling prose can never card as an insertable email.
• body_format gate decoupled: artifact stamping extracted to pure buildOutlookArtifact with two doors (body_format arg OR offerable client actions).
• Confirm-card state machine + auto-prompt one-shot extracted to useClientActionConfirmFlow, shared by the email renderer (behavior unchanged, suite green) and the new OutlookEratoAppointmentRenderer.
• Neutral-context sends record a NO_ITEM_SEND_IDENTITY sentinel so the item-independent appointment action can still auto-prompt from the pinned no-item pane; item-bound reply executors treat it as a mismatch (fail closed).
• Mobile gate: no action on Outlook iOS/Android (displayNewAppointmentForm unsupported); recurrence never prefilled.
• Facet TOML: client_actions + presentation = "auto_prompt" + client_actions_always_ask, step-4 rewrite (ports the shelved 4a/4b/4c baseline; "never claim booked" kept). Content-package counterpart: EratoLab/erato-subscription-content branch feature/ermain-387-create-appointment-facet (byte-identical section, stacks on content#1).

Tests: both-gates proposal validation, artifact-stamping path (confirm surface works without body_format), parser/form-opener (incl. mobile gate + no recurrence), appointment renderer confirm/deny/auto-prompt (incl. neutral-context sentinel match), email-renderer kind filter, MessageContent fence dispatch + carding gates.

• Success feedback follows the add-in's transient idiom (unified 2026-07-06): confirm cards close on every resolution, the action button swaps to "Opened!" for ~2s — no persistent resolved rows. Web/library counterpart tracked as ERMAIN-435.