feature - implement RFC 023 approximate functions (#40)

[dannymeijer]

Summary

• completes RFC 023's portable approximate aggregate surface with approx_count_distinct(...) and approx_percentile(...)
• carries approximate aggregate arguments through Prism/Substrait lowering, including percentile and accuracy for approx_percentile(...)
• adds registry approximation metadata, aggregate modifier policy, Substrait extension mappings, and DataFusion adapter-boundary name rewrites for both helpers
• records the sketch-state decision explicitly: sketch construction/merge/estimate/serialization names are delegated to new Draft RFC 025 instead of being exposed as untyped binary/string functions
• marks RFC 023 Implemented with RFC 025 carrying the follow-up typed-sketch value design
• adds RFC 025 for typed sketch logical values, links it to issue RFC 025: Typed sketch logical values #51, and links RFC 023/reference docs/release notes to that boundary
• updates the RFC, function reference, aggregate-builder docs, and release notes so the PR no longer describes a first slice

Verification

• git diff --check
• make fmt
• incan test tests/test_approximate_functions.incn
• incan test tests/test_function_registry.incn
• incan test tests/test_substrait_plan.incn
• incan test tests/test_prism.incn
• incan test tests/test_session_aggregates.incn
• make fmt-check
• make test-style
• make registry-metadata
• make build
• make test
• make smoke-consumer
• make pre-commit

After the docs-only RFC 025 / RFC 023 status commits:

• git diff --check

Stack / Merge Order

This PR is stacked after #49 and should merge only after:

#44 -> #45 -> #46 -> #47 -> #48 -> #49 -> this PR

Refs #40
Refs #51