feature - add portable plan-validation backend (#22)

[dannymeijer]

Summary

Adds a portable plan-validation backend for IncQL sessions so Substrait plan construction can be validated without using DataFusion as the execution owner. This keeps the architecture aligned with the authoring/query surface -> Prism -> Substrait IR -> backend adapter boundary while providing a non-executing backend path for plan-shape checks.

Closes #22.

Type of change

• Bug fix
• New feature
• Refactor / maintenance
• Documentation
• CI / tooling
• RFC (adds/updates docs/rfcs/*)

Area(s)

Select the primary areas touched (labels sync from checked lines when the triage workflow runs):

• Package & tests
• Specification (RFCs)
• Documentation
• Automation & repo config
• Other

Key details

• User-facing behavior: sessions can opt into Session.plan_validation_backend() / Session.builder().with_plan_validation(...) for validation-only plan construction. The backend accepts valid plans through execute(...) and returns typed backend errors for materialization and write paths.
• Internals: adds BackendKind.PlanValidationEngine, PlanValidationBackendConfig, backend selection encoding/decoding, and a dispatcher path that validates Substrait plan roots without delegating semantics to DataFusion.
• Risks: this backend intentionally does not materialize records or write sinks. Tests cover those paths as explicit errors so callers do not mistake validation coverage for execution support.

Testing / verification

• make ci (or make fmt-check, make build, make test)
• Manual verification described below

Manual verification notes:

• /Users/danny/Development/encero/tmp/incan-v0.4.0-bin/bin/incan --version -> incan 0.4.0
• git diff --check
• make test-style
• make fmt-check INCAN=/Users/danny/Development/encero/tmp/incan-v0.4.0-bin/bin/incan
• make build INCAN=/Users/danny/Development/encero/tmp/incan-v0.4.0-bin/bin/incan
• /Users/danny/Development/encero/tmp/incan-v0.4.0-bin/bin/incan fmt --check src
• /Users/danny/Development/encero/tmp/incan-v0.4.0-bin/bin/incan fmt --check tests/test_session_backend_plan_validation.incn
• /Users/danny/Development/encero/tmp/incan-v0.4.0-bin/bin/incan test tests/test_session_backend_plan_validation.incn -x --nocapture -> 7 passed

Docs impact

• No docs changes needed
• Docs updated
• Docs follow Divio intent (tutorial/how-to/reference/explanation) where applicable

If docs updated:

• Link(s): docs/language/reference/execution_context.md, docs/release_notes/v0_1.md

Checklist

• I kept public docs user-focused and moved internals to contributing docs when appropriate
• I avoided duplicating canonical install/run instructions in multiple places
• I added/updated tests where it materially reduces regressions