N2: Session crossfade (smooth blend on session switch)

[pxlcrtiv]

Vertical slice. As a listener I want switching sessions to crossfade smoothly instead of snapping.

Blocked by: None — can start immediately.

Acceptance criteria:

• Loading/switching a session ramps from the current audio state to the new session's t=0 baseline over a configurable duration (default ~300ms).
• No audible snap on switch.
• Manual (non-sequencer) mode is unaffected.
• crossfade param interpolation is unit-tested (reuse evaluateSession-style assertions).
• npx tsc --noEmit = 0, npx vitest run green, npm run build = 0.

Branch: feat/n2-crossfade (off main). See docs/spec-anahata-next.md (N2).