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

[codecov]

Codecov Report

❌ Patch coverage is 89.50276% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.06%. Comparing base (a042e3c) to head (3e33ee8).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...query_engine/model/series/points_in_time_series.py	89.47%	6 Missing and 2 partials ⚠️
...ery_engine/analyze/query/solvers/default_solver.py	77.41%	5 Missing and 2 partials ⚠️
..._engine/analyze/metadata/time_series_expression.py	93.93%	1 Missing and 1 partial ⚠️
...mpulse_query_engine/analyze/query/query_builder.py	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
+ Coverage   88.99%   89.06%   +0.06%     
==========================================
  Files          61       62       +1     
  Lines        5199     5360     +161     
  Branches      627      648      +21     
==========================================
+ Hits         4627     4774     +147     
- Misses        461      470       +9     
- Partials      111      116       +5     

Flag	Coverage Δ
query_engine	85.07% <89.44%> (+0.30%)	⬆️
reporting	94.25% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
..._query_engine/analyze/query/solvers/blob_solver.py	49.09% <100.00%> (ø)
..._query_engine/analyze/query/solvers/empty_cache.py	88.88% <100.00%> (ø)
...query_engine/analyze/query/solvers/series_cache.py	80.00% <100.00%> (ø)
...uery_engine/analyze/query/solvers/solver_config.py	100.00% <100.00%> (ø)
src/impulse_query_engine/measurement_db.py	79.26% <100.00%> (+3.26%)	⬆️
...impulse_query_engine/model/series/sample_series.py	84.80% <100.00%> (+0.13%)	⬆️
...rc/impulse_query_engine/model/series/value_type.py	100.00% <100.00%> (ø)
src/impulse_query_engine/schema.py	100.00% <100.00%> (ø)
src/impulse_reporting/config/config_parser.py	97.19% <100.00%> (+0.01%)	⬆️
..._engine/analyze/metadata/time_series_expression.py	78.91% <93.93%> (+1.46%)	⬆️
... and 3 more

... and 2 files with indirect coverage changes

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