Feature/custom solver support

[adefabian]

Summary

Customers can now ship their own QuerySolver (say, one that reshapes their raw
tables into the Impulse silver schema) without touching Impulse core.

Solvers register themselves by name, the report config picks one by that name through
the existing query_engine.solver field, and each solver can bring its own
SolverConfig subclass whose extra fields get checked at parse time.

Config picks a solver by registered name, not by class path. So a config can't
make Impulse import and run some arbitrary class. What solvers exist depends
entirely on what the driver imports.

DefaultSolver moves onto this mechanism with no behavior change. It registers
under "DefaultSolver" (plus the old "DeltaSolver" / "KeyValueStoreSolver"
aliases), so existing configs keep working.

Changes

• Registry (solvers/registry.py): @register_solver(name, config_cls, *, aliases, overwrite) plus resolve_registration / is_registered / registered_names. Maps a name to (solver_cls, config_cls), treats re-registering the same class as idempotent, rejects conflicting duplicates, and lists the known names
  when a lookup misses.
• Uniform construction (QuerySolver.from_config + solvers/solver_context.py): a frozen SolverBuildContext (spark, solver_config, is_raw_data, drop_implausible_data, raw_encoder) is the one place construction inputs live. Base from_config passes just solver_config; DefaultSolver overrides it to also
  wire spark and the raw-data flags. The report factory builds any solver the same way, whatever its constructor looks like.
• Config (config_parser.py): query_engine.solver is a plain str now (any registered name). Solvers becomes a StrEnum so old == Solvers.DEFAULT_SOLVER comparisons still hold. A mode="before" validator re-validates solver_config through the selected solver's config_cls, so a custom solver's fields are
  enforced at parse time and an unregistered name fails early as a ValidationError.
• Report factory (core/report.py): create_solver resolves the name through the registry and calls from_config(ctx) instead of a hardcoded match. Solver telemetry now logs the registered name string rather than an enum member.
• Docs (docs/impulse/docs/data_model/ingestion.md): a "Registering a custom solver" section covering subclassing DefaultSolver, declaring a SolverConfig subclass, selecting it by name, and importing the package so registration runs — plus a worked column-redaction example.
• Skills (skills/impulse-config, skills/impulse-data-model): updated to describe selecting a solver by registered name.

Customer solvers extend behavior by overriding the supported QuerySolver
pipeline methods (e.g. solve); the report factory and incremental
change-detection paths are unchanged and continue to read silver data through
MeasurementDB.

Test Plan

• Unit tests added/updated — registry (registry_test.py), from_config hook (from_config_test.py), config/solver-selection validation (solver_registry_config_test.py)
• Integration tests added — report-factory routing (custom_solver_report_test.py) and an end-to-end custom-solver solve (custom_solver_test.py)
• Documentation updated

Checklist

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

[codecov]

Codecov Report

❌ Patch coverage is 97.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.89%. Comparing base (83838ee) to head (a9a2bfb).

Files with missing lines	Patch %	Lines
src/impulse_reporting/config/config_parser.py	88.23%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #68      +/-   ##
==========================================
+ Coverage   88.69%   88.89%   +0.20%     
==========================================
  Files          60       62       +2     
  Lines        5015     5081      +66     
  Branches      598      602       +4     
==========================================
+ Hits         4448     4517      +69     
+ Misses        461      459       -2     
+ Partials      106      105       -1     

Flag	Coverage Δ
query_engine	84.91% <100.00%> (+0.34%)	⬆️
reporting	94.29% <91.30%> (+0.08%)	⬆️

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

Files with missing lines	Coverage Δ
...ery_engine/analyze/query/solvers/default_solver.py	97.68% <100.00%> (+0.70%)	⬆️
...query_engine/analyze/query/solvers/query_solver.py	88.40% <100.00%> (+0.52%)	⬆️
...lse_query_engine/analyze/query/solvers/registry.py	100.00% <100.00%> (ø)
...ery_engine/analyze/query/solvers/solver_context.py	100.00% <100.00%> (ø)
src/impulse_reporting/core/report.py	92.64% <100.00%> (+0.97%)	⬆️
src/impulse_reporting/config/config_parser.py	96.29% <88.23%> (-0.99%)	⬇️

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