viewer: rename internal spawnAssertEvalRun / resolveAssertEvalCommand symbols to *AssertAi*

[tangym]

Context

After the assert-eval → assert-ai package rename (#177), a couple of internal
TypeScript symbols in the viewer still carry the legacy AssertEval name.
They're internal-only — no public API, no env-var, no behavior change.

Symbols to rename

viewer/src/lib/server/run-spawn.ts:

• function resolveAssertEvalCommand(...) → resolveAssertAiCommand (def + internal call site)
• export function spawnAssertEvalRun(...) → spawnAssertAiRun (def + docstring example)

viewer/src/routes/api/runs/+server.ts:

• import { spawnAssertEvalRun } ... → spawnAssertAiRun
• call site → spawnAssertAiRun

Total: 2 files, ~6 lines.

Why a separate issue

The viewer is a separate SvelteKit app — keeping this rename isolated lets it
ride with a viewer-side test pass instead of being bundled into unrelated work.

Acceptance criteria

• grep -rn "AssertEval\|assertEval" viewer/src/ returns zero matches
• Viewer builds and lints clean
• Manual smoke: create a run from the viewer wizard and confirm assert-ai run --config ... is spawned and the run appears in the dashboard

Out of scope

• Any behavior change
• Env-var renames
• Anything outside viewer/src/