Maestro compat: support `optional` on scrollUntilVisible and extendedWaitUntil (command- and element-level)

[thymikee]

Context

optional: true is the most common Maestro flake-tolerance idiom (upstream's own e2e fixtures lean on it heavily — see the #1217 parity audit). The engine already has the shared interpreter boundary for it (suppression gated on the command-failure classification, cancellation/infra/ambiguity non-suppressible), currently wired for tapOn and assertion targets. This extends that boundary to scrollUntilVisible and extendedWaitUntil — extension, not new machinery.

Upstream semantics to match

Upstream accepts optional at both positions exercised by the conformance corpus flow:

• command level: scrollUntilVisible: { ..., optional: true } / extendedWaitUntil: { ..., optional: true }
• element level: scrollUntilVisible: { element: { id: ..., optional: true }, ... }

A timed-out optional lookup downgrades to a warning and the flow continues (warned-vs-failed classification, same as tapOn).

Repro (mechanical, via the conformance oracle from #1289)

• Corpus flow: scripts/maestro-conformance/corpus/upstream/076_optional_assertion.yaml
• Declared divergence: FLOW_DIVERGENCES['upstream/076_optional_assertion'] in scripts/maestro-conformance/expected-divergence.ts

Note: flow 076 also uses assertTrue, tracked separately — after this issue lands, the divergence entry narrows to assertTrue only (or the flow splits in two).

Acceptance

• Parser accepts optional at both positions on both commands; interpreter classifies a lookup timeout as warned, flow continues.
• The unsupported list in the 076 divergence entry no longer contains the optional items (entry narrowed or removed); pnpm maestro:conformance green with no new divergences.

[thymikee]

Triage: valid, bounded Maestro-compat enhancement and AFK-ready. Reuse the existing interpreter optional-command suppression boundary; do not add command-local catch-and-warn logic. Normalize command-level and element-level optional into one typed intent for scrollUntilVisible and extendedWaitUntil, and preserve the existing non-suppressible classes (cancellation, infrastructure/provider failure, and ambiguity). Acceptance should include parser vectors for both positions, interpreter tests proving only lookup timeout/miss downgrades to warned, and conformance-oracle narrowing for upstream/076 without masking its separate assertTrue divergence. Labeled enhancement + ready-for-agent.