Ultrafast collapsed into ordinary Fast across the stack: the catalog only
reported `fast`, presets and drafts stored a boolean, session.create took
only `fast`, and Settings had a single Fast switch, so an Ultrafast default
read as Fast and a new chat could lose it.
- Inventory reports `ultrafast` per model from the same route-aware
resolver that builds the request; Daybreak aliases offer no accelerated
speed (their priority request is ignored).
- session.create accepts an exact `service_tier` that wins over the legacy
`fast` flag; unknown tiers are rejected.
- Desktop keeps the exact tier through presets, draft creation, session
options and labels; an unsupported remembered tier falls back to
Standard, and failed edits reconcile to the confirmed state.
- Settings → Model → Main model offers one Speed choice (Standard / Fast /
Ultrafast) on models that support Ultrafast; others keep the Fast switch.
Split out of #129251 (which keeps Daybreak).
(cherry picked from commit 4e332fb850c989ef9f34bd70aaca174042beecc6)
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