feat(ir): add AggIntent::Extension for deployment-model-specific intents

[zzylol]

Summary

Closes/addresses #131 for the L3 vocabulary specifically. Adds a small,
generic escape hatch to AggIntent:

Extension {
    ext_kind: String,             // deployment-model-chosen tag
    #[serde(default)]
    payload: serde_json::Value,   // opaque to core
},

Rather than growing AggIntent for every capability a single deployment
model needs -- which turns the shared vocabulary into a dumping ground,
against the "core only grows for intents ≥2 deployment models actually
use" principle -- a deployment model that needs something core doesn't
have carries it here and owns all interpretation of payload itself.

Core's own methods treat it opaquely and conservatively:

• requires() → DataModel::Any
• is_per_series() → false
• input_col() → None
• output_column() names the column after ext_kind, unconstrained type
• plan::boundary::realize() → Realization::PassThrough

Field is named ext_kind, not kind -- kind is already the enum's own
internal serde tag (#[serde(tag = "kind")]) and collides.

Motivation

ASAPQuery-backend's control_plane is mid-way through merging its own
AggIntent onto this crate's version (their Phase 1:
ProjectASAP/ASAPQuery-backend#391). One of their intents --
Frequency { accuracy }, a standalone point-frequency-via-CMS query
(count(*) WHERE key = k) -- has no equivalent here and isn't the same
thing as this crate's RankingMeasure::Frequency (which classifies what
a TopK ranks by, not a queryable intent on its own). Adding a
control_plane-specific variant directly to this enum would be exactly the
"dumping ground" problem; Extension lets them carry it through the
shared type without that.

This isn't the general inventory-based registration API #131 sketches
for the PromQL parser's function list (L1) -- that's a bigger, separate
design (Rust's exhaustive-match enums don't compose with a runtime
registry the way a lookup table does). This is a minimal, compatible
first step at L3 specifically.

Test plan

• cargo build --workspace -- clean
• cargo test --workspace -- all passing (0 failures across every
  crate)
• rustfmt applied to changed files

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