Support Grafana advanced Expressions (Math / Reduce / Resample / Classic condition)

## Problem

Grafana alerts have two shapes:

- With **Advanced options off**, the rule is a single **"WHEN `[reducer]` OF query IS ABOVE `N`"** — one reducer + one threshold. This is the common case and migrates today.
- With **Advanced options on**, the **Expressions** section lets users chain steps — **Math**, additional **Reduce** steps, **Resample**, **Classic condition** — and the migration parks any such alert for manual review and **does not create it**.

## Impact

Hand-built Grafana alerts that use Advanced options (Math, Classic condition, multi-step chains) don't migrate and must be rebuilt by hand.

## Steps to reproduce

1. Create an alert, enable **Advanced options**, and add a **Classic condition** (or a **Math** step).
2. Migrate it.

**Current behavior:** parked for review, not created.

## Scope of translation

- **Math** (arithmetic/logic across steps) → expressible as arithmetic in the query. *Translatable.*
- **Classic condition** (legacy reduce + threshold in one) → expressible as aggregate + threshold. *Translatable.*
- **Reduce** variants beyond the simple form → aggregation in the query. *Translatable.*
- **Resample** (re-align a series to a fixed step) → **no Kibana equivalent.** *Not translatable — flag and warn.*

## Acceptance criteria

- [ ] Math and Classic-condition alerts migrate to a working rule.
- [ ] Multi-step Reduce chains migrate where the operations have query equivalents.
- [ ] Resample (and any other untranslatable step) is clearly flagged with a reason instead of being silently parked.

Start by locating the Grafana alert migration and expression-translation logic, then inspect how advanced Expressions are currently parked for manual review. Verify that Math, Classic condition, and supported multi-step Reduce chains migrate to working rules, while Resample and other unsupported steps are flagged with clear reasons.

Written by the indexing model from the issue text.