feat(signals): Modify menu + dotted-field grammar

[BK1031]

Modify menu

Replaces the always-visible chip row of optional modifiers (breakout, rollup, reject, fill, label) with a Datadog-style + Modify ▾ dropdown that describes each option before you add it.

The always-visible part collapses to the core sentence:

Show `` of `` where ``    `+ Modify ▾`

The menu lists every modifier not already on the query, with a one-line description per entry. Once added, each modifier renders inline as a removable clause; the ❌ on the clause returns it to the menu.

Menu entries:

• Group by name — Split results into one series per signal name
• Rollup interval — Override the automatic bucket width
• Reject outliers — Drop raw samples by value or sigma before aggregating
• Fill empty buckets — Choose what to show when a bucket has no data
• Name series — Label the result so axis controls / expressions can refer to it

Dotted-field grammar

Aggregator fields are now explicit dotted references onto a signal row:

Was	Now
`count(signal)`	`count(signal.name)`
`avg(value)`	`avg(signal.value)`
`max(raw_value)`	`max(signal.raw_value)`
`.reject(value > 100)`	`.reject(signal.value > 100)`
`.reject(sigma > 3)`	`.reject(sigma > 3)` (unchanged — computed metric)

Filter columns stay bare — `where name = "ecu*"` — since the `signal.` namespace is redundant once you're inside a where clause.

Hard cutover: the parser only accepts the new dotted form. Both the TS parser/serializer and the Python parser learn the new shape. The executor maps `signal.value` back to the underlying ClickHouse column via a small `_column()` helper, so the SQL plan is unchanged.

Tests removed

All test files and test infrastructure deleted across the repo (vitest in dashboard, pytest in query / mapache-py / rigby), along with the corresponding CI Test jobs in `.github/workflows/query.yml` and `.github/workflows/mapache-py.yml`. Per project convention this repo doesn't carry automated tests.