feat(plan): pluggable AggIntent::Extension realization + readout hook (closes #150)

[zzylol]

Summary

Closes #150: boundary::implementation_for_with mapped every AggIntent::Extension unconditionally to Implementation::PassThrough, with no way for a deployment to opt in to its own realization for a shape core doesn't know (e.g. control_plane's Extension{"frequency"} point-frequency intent — count(*) WHERE key = k).

Two new CostModel methods, both with safe defaults so every existing deployment is unaffected:

• realize_extension(ext_kind, payload) -> Implementation — consulted by implementation_for_with's Extension arm instead of hardcoding PassThrough. Default: PassThrough (today's behavior).
• readout_extension(ext_kind, payload, col) -> SketchQuery — consulted by bind::readout's new Extension arm. This half wasn't in boundary::implementation_for maps AggIntent::Extension to PassThrough unconditionally — no deployment-specific realization hook #150's literal ask, but turned out to be required for the feature to work end-to-end: readout() previously had other => unreachable!(...) for anything besides Quantile/Cardinality/TopK/Count — a CostModel that realizes an Extension as Sketch via realize_extension alone would panic the first time readout() actually fires for it. Default panics loudly with a clear message pointing at the missing override, rather than silently misinterpreting payload.

Also adds SketchQuery::PointCount.value: Option<String> — needed by readout_extension to carry a per-item filter value (e.g. count(cms_metric{item="checkout"}) → PointCount{key: Named("item"), value: Some("checkout")}). SketchQuery is a closed, exhaustively-matched enum with no room to grow this otherwise, and no SummaryExecutor implementation has any way to resolve a filter value on its own — readout()'s trait signature has no tree/QueryExpr access at all, only the already-decoded state + query.

This is prep work for ASAPQuery-backend's data_plane to close its own SketchQuery::PointCount named-key gap (companion to the SketchQuery::TopK gap closed in ASAPQuery-backend#412) — control_plane will implement both new methods for its Extension{"frequency"} intent in a follow-up PR once this merges.

Test plan

• cargo test -p asap-plan -p asap-sketch — 39 tests, all passing, including 2 new: extension_intent_stays_logical_by_default (regression check — an Extension intent with no CostModel override still stays Logical, unchanged) and extension_intent_binds_via_custom_cost_model (a custom CostModel overriding both hooks binds a real SummaryAgg/SummaryEstimate with the expected SketchKind/SketchQuery::PointCount{key, value} instead of Logical/a panic).
• cargo test --workspace — full workspace, 400+ tests across all crates, all passing.
• cargo build --workspace — clean.
• cargo clippy --workspace --all-targets — zero warnings.
• rustfmt scoped to the touched files.

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