Stage 1: BaseConfig extends state.Provider + Option F 5-sub-point contract

Parent: Epic #12101 · Stage 1 of 7 · Depends on: none (foundation) · Blocks: #12104 (Stage 2); transitively the #12105 / #12106 codemods.

What

Build a new ai/BaseConfig.mjs that extends Neo.state.Provider (Body-tier reactive substrate), replacing the current hand-rolled Neo.core.Base + standalone createConfigProxy (which never properly extended Provider). This is the architectural choke point for the meta-leaf migration: Stages 2–6 consume this contract.

The 5-sub-point Option F contract (epic AC-Epic-3)

1. set(path, value) — explicit mutation API; routes through Provider's setData(path, value) → internalSetData (Provider.mjs:497, the single write funnel).
2. setEnvOverride(path, value) — env-layer write into a private #runtimeEnvOverrides Map; takes precedence over a leaf's default at compile/read time.
3. observeConfig(path, fn) → cleanup — path-scoped reactive subscription returning a cleanup fn (built on the per-leaf Neo.core.Config.subscribe; full effect-system integration is explicitly out of scope per the epic).
4. Direct assignment via Provider's set trap — aiConfig.data.x.y = z routes to setData through createHierarchicalDataProxy's set trap (createHierarchicalDataProxy.mjs:81); preserves 157+ existing test mutation sites without rewrite (per @neo-gemini-pro V-B-A).
5. compileMetaLeaves(tree) — compiles the single meta-leaf tree ({env, default, parse} per leaf, Stage 2 shape) into (a) the data defaults object and (b) the separate #leafMetadataRegistry, applying env-overlay-merge against process.env + #runtimeEnvOverrides.

Acceptance Criteria

• AC1 — extend, don't reinvent (AC-Epic-1): BaseConfig extends Neo.state.Provider; no parallel reactivity machinery. Reuse Neo.util.Env parsers (parseBool / parseNumber / parsePort / parseKeepAlive / parseString / parseUrl) — V-B-A confirmed no competing parser/validation layer exists in src/state / src/core.
• AC2 — parser-validation at the internalSetData() choke point (AC-Epic-5): override internalSetData with the metadata-path-first rule — validate/coerce ANY value at a known leaf path (looked up in #leafMetadataRegistry); namespace-recursion intermediates pass through unchanged.
• AC3 — separate #leafMetadataRegistry (AC-Epic-5): the parser/env-var metadata registry is independent of Provider's reactive #dataConfigs (full-dotted-path → Config). Two registries, distinct responsibilities.
• AC4 — hierarchical deep-merge (AC-Epic-5): via Provider's existing data_: {[isDescriptor]:true, merge:'deep', value:{}} descriptor — no custom merge.
• AC5 — Body-tier lineage cited (AC-Epic-4): the implementation PR cites #6131 (base class) / #6965 + #6973 (createHierarchicalDataProxy) / #6974 (Effect reactivity) / #6981 (set-trap→setData) / #6985 (deep-merge data_).
• AC6 — Option F mutation surface (AC-Epic-3): all 5 sub-points implemented + unit-tested, including a direct-assignment test that proves the set-trap path.

Contract Ledger

Dependency chain (for downstream tickets)

• #12104 (Stage 2) consumes the compileMetaLeaves meta-leaf tree shape from this ticket's PR (number TBD on open).
• #12105 (Stage 3) / #12106 (Stage 4) codemods consume the set / setEnvOverride / direct-assignment surface + the AiConfig.X.Y read path from this ticket's PR.

Discussion #12100 criteria linkage

Stage-1 ACs trace to Discussion #12100 §9 Criterion 4 (OQ9 21-leaf prototype validated compileTree end-to-end) + the three STEP_BACK-convergence Stage-1 ACs (parser-validation choke point + separate metadata registry + hierarchical deep-merge), preserved per Discussion #12100's ## Discussion Criteria Mapping.