Feature/poi support

[adefabian]

Summary

Adds Points-in-Time (POI) series as a first-class silver-layer channel type:
a value defined only at its timestamp (no between-point validity), backed by
PointsInTimeSeries, sitting alongside the existing interval-valued SampleSeries.
The motivating case is ECU Diagnostic Trouble Codes (DTCs) — string-valued fault
events analyzed together with continuous signals ("RPM at the instant DTC == P0301").

Changes

• Storage: new poi_channels silver table (POI_CHANNELS_SCHEMA); series type is
  determined by table membership (channels ⇒ SAMPLE, poi_channels ⇒ POI) — no
  series_type column needed.
• Model: PointsInTimeSeries gains string-value support — == / != and sampling
  for strings; arithmetic/ordering/reductions raise for strings via a @_numeric_only
  guard; value-type-aware dtype().
• Query API: QueryBuilder.poi_channel(dtype=...) (accepts the enum or the plain
  string "double"/"string"); the selector carries series_type + value_type and
  is the plan-time source of truth for series-type dispatch.
• Solve stage: _prepare_channels_join unions poi_channels after RLE encoding
  (zero-duration points aren't merged); TimeSeriesCache.load_blob builds the right
  series from the selector; a solve-time assertion rejects a declared-vs-actual mismatch.
• Config/DB: poi_channels_uri wired through MeasurementDBConfig,
  MeasurementDB.poi_channels(), and the reporting Source config.
• Tests: POI integration suite (freeze-frame, string equality + op gating,
  mix-and-match, declared-vs-actual, backward-compat), PointsInTimeSeries string
  units, poi_channel(dtype=...) coercion units, and a config-passthrough regression.
• Demo: reporting_pipeline.ipynb gains a DTC section — freeze-frame (RPM at each
  P0301 misfire), per-recording fault counts, and a ±10 s window histogram around faults.
• Docs: API reference regenerated; make update-api-docs target added.

Test Plan

• Unit tests added/updated
• Manual testing completed
• Documentation updated (if applicable)

Checklist

• Code follows project style guidelines
• Self-review completed
• No new linter warnings introduced