Add pure weekly period bundle binding contract

docs/weekly_period_bundle_contract.md

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+# `pert.weekly.period_bundle.v1`
+
+This is a pure contract library for a later trusted Actions acquisition
+harness. It performs no filesystem, GitHub API, upload, download, or producer
+work. The existing `pert.weekly.period_lock.v1` remains the period source of
+truth.
+
+## Immutable models
+
+- `BundleContext` fixes the original `run_id`, `source_attempt=1`, exact
+  reviewed workflow SHA, producer SHA, expected period lock, and external
+  artifact evidence (`name`, `id`, digest, retention).
+- `BundleWire` contains only canonical `period_lock.json`,
+  `input_snapshot.json`, and `bundle_manifest.json` bytes plus the external
+  artifact evidence. Its SHA-256 properties cover all three byte members.
+- `RerunContext` accepts only current `run_attempt=2` and reuses the original
+  context; it requires the same `run_id`, repository, and reviewed workflow
+  SHA as the original context and cannot derive a new period or snapshot.
+
+The repository is an explicit `owner/name` binding in the manifest. Untrusted
+bundle members are bounded before parsing: lock 64 KiB, snapshot 256 KiB,
+manifest 64 KiB, snapshot depth 16, object keys 64, list items 256, strings
+4096 characters, and source artifacts 64. Exceeding any bound fails closed.
+
+The canonical manifest binds the bundle version, artifact name, original run
+identity, workflow and producer SHAs, lock/snapshot versions, and exact lock
+and snapshot hashes. Artifact id/digest/retention are external Actions
+evidence and are required to match the immutable `BundleContext`; manifest
+self-hashing is intentionally avoided.
+
+## Verification rules
+
+`verify_period_bundle()` reconstructs the expected lock, snapshot, and manifest
+from the complete context and requires byte-for-byte equality. It does not
+accept a manifest recomputed from a replacement lock or snapshot. Duplicate,
+unknown, missing, reordered, malformed, noncanonical, unsafe-integer, wrong
+run/attempt/SHA/artifact, and tampered values fail closed with sanitized
+`WeeklyBundleError` codes. `verify_bundle_collection()` requires exactly one
+bundle. Unexpected programming/system exceptions are not broad-caught.
+
+This library is PR-1A only. A privileged workflow that has `actions:read` must
+be a later PR sourced from the trusted default branch and must never execute
+PR-controlled code.