fix(grafana): translate named seriesOverrides stack groups

[stefans-elastic]

Closes #446

Summary

Grafana's seriesOverrides[].stack is tri-state: false, true, or a group name ("A", "B", "D"…). Only false was honored, so a named group fell through and a series Grafana draws as a separate overlay got accumulated onto the stacked breakdown it is meant to sit above.

The case

Dashboard 9852's Memory panel:

{"alias": "/Total memory/", "color": "#bf1b00", "fill": 0, "linewidth": 2, "stack": "D"}

Grafana stacks active+buffers+cached to 6.46 GB and draws Total memory as its own unfilled line at 6.20 GB. The migration stacked all four:

	Grafana	Kibana (before)
stacked band	6.46 GB	6.45 GB
overlay line	6.20 GB	6.20 GB
chart maximum	6.46 GB	12.65 GB

Every individual series value was already correct — only the stacking composition was wrong, which makes a side-by-side look like a data discrepancy when it isn't.

Approach

A named group holding exactly one series is an unstacked overlay, and it maps losslessly onto machinery that already existed end-to-end:

• the panel schema documents per-metric stack — "rendered as an unstacked overlay"
• the Kibana target already emits a stacked layer plus an unstacked overlay layer (dashboards_api.py, commented "kubernetes-mixin style: stack usage by breakdown, overlay requests/limits unstacked")

Only the source-side mapping was missing. Resulting Memory panel:

Lens layer	Series
area_stacked	Memory active, Memory buffers, Memory cached
line	Total memory

Groups with 2+ members are disclosed, not reshaped. Lens accumulates one stack per layer, so several independent stacks cannot be represented; those stay stacked and emit a warning rather than being silently restructured.

Reviewer note

The fix only takes effect when the series name matches the override alias. 9852's alias is the regex /Total memory/ (with a space) while default naming yields Total_memory (underscore), so it matches there only because the curated pack renames the series. Underscore-vs-space alias matching for un-packed dashboards is a separate gap, flagged at the end of #446 — not addressed here.

Test plan

• test_named_stack_group_singleton_becomes_unstacked_overlay — singleton → overlay, siblings stay stacked, and the native output carries both an area_stacked and an unstacked layer
• test_named_stack_group_with_multiple_series_is_disclosed_not_reshaped — 2-member group stays stacked and surfaces a disclosure
• make test 6390 passed, 61 skipped; make lint and make typecheck clean
• Verified in a browser against Grafana: chart maximum 6.45 GB vs Grafana's 6.46 GB

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