Add the runtime heart of extension points: `ModuleSource::resolve_policy`
decides lexical vs dynamic resolution by the NEAREST declaration of a name
walking up the importing origin's chain — a concrete module → lexical (seal
to origin, Phase 4b); an extension-point marker → dynamic, resolved against
the inheriting app (`cx.app_id`) so the app can override, falling back to the
default body up-chain; the EP wins a depth tie.
- shared: `ModuleResolution { Module | NoProvider | NotFound }` + a
`resolve_policy(origin, inheriting_app, name)` trait method with a
lexical-only default impl (existing fakes inherit it unchanged).
- PostgresModuleSource: one-round-trip nearest-declaration query (min EP depth
vs min module depth over the chain CTEs), then the lexical/dynamic branch.
- module_resolver: calls `resolve_policy(origin, cx.app_id, path)`; maps
NoProvider → a clear "extension point has no provider" error, NotFound →
module-not-found. Cache + AST-source sealing unchanged.
- executor-core tests: a policy fake + 3 cases (app override binds dynamically,
no-provider errors, non-EP stays lexical).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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