Fetch snapshots during join, rather than at startup

[eddyashton]

This was a 'mare to land. Will add some comments inline.

The goal was to push snapshot fetching later in the startup process, and late enough that we can do it when receiving a StartupSeqnoIsOld error response to a /join.

While doing that, I've decoupled the verification from the deserialisation of the snapshots, in a hopefully readable way, alongside a bunch of the helper functions for accessing snapshots.

[Copilot]

Pull request overview

Shift snapshot fetching later in the node startup flow so joiners only fetch a newer snapshot after receiving a StartupSeqnoIsOld response, while refactoring snapshot handling to separate receipt verification from snapshot deserialisation.

Changes:

• Move join-time snapshot discovery/fetching into the enclave join path (triggered on StartupSeqnoIsOld) rather than doing it at host startup.
• Refactor snapshot serdes into segment separation + explicit receipt verification, and add helpers to enumerate committed snapshots across directories.
• Update snapshot lookup/redirect behaviour, tests, and operator documentation to reflect the new flow.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
tests/reconfiguration.py	Adjusts join test to exercise redirected snapshot discovery and snapshot absence scenarios.
src/snapshots/snapshot_manager.h	Simplifies latest-snapshot lookup API to return a single path from multiple directories.
src/snapshots/filenames.h	Adds helpers to list/sort committed snapshots across directories and select the newest.
src/snapshots/fetch.h	Updates peer snapshot fetch to use in-memory CA blob rather than a CA path.
src/node/snapshot_serdes.h	Splits receipt verification from snapshot application (segment separation + verify + deserialise).
src/node/rpc/file_serving_handlers.h	Redirects snapshot requests using the snapshot filename rather than a full path.
src/node/node_state.h	Adds join-triggered background snapshot fetching and local snapshot discovery/verification during join/recover.
src/host/test/ledger.cpp	Updates tests for the new snapshot-manager return type (path semantics).
src/host/run.cpp	Removes host-side snapshot preload/fetch, and forwards join snapshot-fetch settings into StartupConfig.
src/enclave/main.cpp	Updates enclave create entrypoint signature (no startup snapshot payload).
src/enclave/entry_points.h	Updates enclave create entrypoint declaration accordingly.
src/enclave/enclave.h	Updates enclave node creation call signature accordingly.
include/ccf/node/startup_config.h	Adds join snapshot-fetch config fields to StartupConfig::Join.
doc/operations/ledger_snapshot.rst	Documents the new join flow (including flowchart) and revised snapshot-fetch timing.