feat: support snapshot

[kerthcet]

What this PR does / why we need it

Which issue(s) this PR fixes

part of #6

Special notes for your reviewer

Does this PR introduce a user-facing change?

[Copilot]

Pull request overview

This PR introduces an initial “Git-inspired” snapshot subsystem in the sandd daemon crate (content-addressable object store + tree metadata + snapshot manager), along with supporting docs, examples, and benchmarks. It also reshuffles documentation links to point at new docs/proposals/* specs and adds a sandd library target to expose the snapshot API to examples/benches.

Changes:

• Add a new sandd::snapshot module (object store, tree model, snapshot manager, types) with async create/restore/list/get/delete operations.
• Add snapshot examples and Criterion benchmarks; add a Makefile target to run/view benchmark results.
• Add proposal docs (Tunnel/Protocol/Snapshots) and update repo documentation links to point to the proposals.

Reviewed changes

Copilot reviewed 17 out of 20 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
sandd/src/snapshot/types.rs	Defines Snapshot/SnapshotInfo metadata types.
sandd/src/snapshot/tree.rs	Defines tree entry format + mode/mtime helpers.
sandd/src/snapshot/object_store.rs	Implements BLAKE3 content-addressable blob store + tests.
sandd/src/snapshot/manager.rs	Implements snapshot creation/restoration + snapshot listing APIs + tests.
sandd/src/snapshot/mod.rs	Exposes snapshot submodules and re-exports key types.
sandd/src/main.rs	Exposes snapshot module from the binary crate; simplifies command execution path.
sandd/src/lib.rs	Adds a library crate entrypoint exporting snapshot.
sandd/Cargo.toml	Adds library target, examples, benchmarks, and snapshot-related deps.
sandd/benches/snapshot_bench.rs	Adds Criterion benchmarks for snapshot create/restore scaling.
examples/snapshot_simple.rs	Adds a simple end-to-end snapshot demo.
examples/snapshot_real_project.rs	Adds a “real repo” snapshot demo using git clone.
README.md	Updates documentation links to proposal docs.
Makefile	Adds benchmark / benchmark-results targets.
hack/docker/README.md	Updates tunnel doc link to proposals path.
examples/tunnel-simple/README.md	Updates tunnel doc links to proposals path.
docs/proposals/TUNNEL.md	Adds tunnel mode proposal documentation.
docs/proposals/SNAPSHOTS.md	Adds snapshot proposal/design documentation.
docs/proposals/PROTOCOL.md	Adds protocol proposal/spec documentation.
docs/DEVELOP.md	Updates protocol spec link to proposals path.
Cargo.lock	Locks new dependencies for snapshot/bench/proposal additions.

Comments suppressed due to low confidence (1)

Makefile:145

• benchmark-results uses open, which is macOS-specific and will fail on Linux/Windows. Prefer a portable xdg-open/open fallback or print the path for manual opening.

.PHONY: benchmark-results
benchmark-results:
	@echo "Benchmark results for sandd daemon:"
	@echo ""
	open target/criterion/report/index.html

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[kerthcet]

/lgtm

[kerthcet]

/kind feature

[InftyAI-Agent]

Approved: PR has both lgtm and approved labels