Update batch collection documentation

docs/internal/collect.md

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+# Collection Internals
+
+This document describes the app-facing upstream collection boundary in
+`internal/collect`.
+
+## Purpose
+
+`internal/collect` is the canonical package used by app workflows to collect
+upstream Weather API data. It constructs the Weather API adapter, fetches a
+normalized bundle, and returns that bundle without applying report selection or
+batch policy.
+
+## Contract
+
+Inputs:
+
+- `collect.Request`, containing the effective `config.Config`
+- `context.Context` for cancellation
+
+Output:
+
+- `collect.Result`, containing `*weatherdata.Bundle`
+
+`Run` returns an actionable error when Weather API adapter construction or
+bundle fetch fails. The package does not derive `facts.CollectedFacts`, build
+modules, resolve report periods, select reports, write state, invoke
+Scriptorium, or notify distributor.
+
+## App Usage
+
+`internal/app` owns a narrow `Collector` interface for orchestration tests. The
+default implementation calls `collect.Run`.
+
+Single-report generation collects once, resolves the requested report, and
+passes the explicit collection into report generation. Batch generation
+collects once before planning and passes the same collection into each planned
+report. If collection returns no bundle, app orchestration returns an error
+before report generation.
+
+## Boundaries
+
+Weather API HTTP details stay in `internal/adapters/weatherapi`. The collection
+package returns normalized `weatherdata` only. It must not know about report
+IDs, prompt IDs, batch names, Daily eligibility, module composition, Recent
+Changes, state paths, or Scriptorium arguments.
+
+## Tests
+
+Inspect:
+
+- `internal/collect/collect_test.go`
+- `internal/app/app_test.go`
+
+## Invariants
+
+- App-facing Weather API collection goes through `internal/collect`.
+- Collection returns normalized source data, not report facts or prompt input.
+- Report and batch policy belongs outside `internal/collect`.