refactor: single source of truth for SYNC_BATCH_LIMIT (closes #207)

[intendednull]

Hoists the sync-batch cap into willow-common::SYNC_BATCH_LIMIT and deletes the two mirror constants in storage and client. Behaviour unchanged (still 10_000); the change is purely "single source of truth, ship".

Placement

Constant lives in crates/common/src/lib.rs. willow-common is the workspace-wide leaf that already owns WireMessage::SyncBatch (the wire type the cap bounds) and is already a dep of willow-client. Adding it as a dep of willow-storage is a one-line cost; both crates now import the same pub const from the same place.

What changed

• crates/common/src/lib.rs — new pub const SYNC_BATCH_LIMIT: usize = 10_000; with a doc comment that names both call sites and explains why they MUST agree.
• crates/storage/src/store.rs — drop the associated const SYNC_BATCH_LIMIT on StorageEventStore, drop the // TODO(#207) mirror-warning comment, switch Self::SYNC_BATCH_LIMIT / StorageEventStore::SYNC_BATCH_LIMIT to the imported free constant.
• crates/client/src/listeners.rs — drop the local const MAX_SYNC_BATCH_SIZE, use SYNC_BATCH_LIMIT from common (also unifies the name).
• crates/storage/Cargo.toml — add willow-common dep.

Tradeoffs considered

• Workspace [env] var or build-script knob. Rejected — not a real Rust constant, requires option_env! parsing, and the cap is a protocol invariant (producer must not exceed validator), not a deploy knob.
• New willow-limits crate. Rejected — one constant doesn't justify a crate; willow-common is already the right home.
• Promote to ClientConfig. Rejected for this PR — both producer (storage) and consumer (client) need to agree at compile time, so runtime tunability would require a coordinated wire-level negotiation. That's a separate feature; this PR closes the "drift hazard" bug today.

Verification

• cargo check -p willow-common -p willow-storage -p willow-client — clean.
• cargo clippy -p willow-common -p willow-storage -p willow-client --all-targets -- -D warnings — clean.
• cargo test -p willow-storage — 33/33 pass, including history_caps_caller_limit_to_sync_batch_limit which directly exercises the cap.

Closes #207.

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Generated by Claude Code

[intendednull]

single source of truth please, the best way you see fit

[intendednull]

Done. Picked willow-common as the home: it already owns WireMessage::SyncBatch (the wire type this cap bounds) and was already a dep of willow-client, so adding it to willow-storage (one line) keeps the constant colocated with the type it constrains.

• crates/common/src/lib.rs — pub const SYNC_BATCH_LIMIT: usize = 10_000;
• crates/storage/src/store.rs — dropped the associated const + TODO mirror-warning, imports from common
• crates/client/src/listeners.rs — dropped MAX_SYNC_BATCH_SIZE, imports from common (also unifies the name)

Behaviour unchanged. cargo check, clippy -D warnings, and cargo test -p willow-storage (33/33) all clean. Title and body updated, draft flag flipped, PR is ready for review.

Tradeoffs in the PR body — short version: rejected option_env! workspace var (not a real const, and this is a protocol invariant not a deploy knob), rejected a new willow-limits crate (overkill for one constant), and intentionally deferred ClientConfig promotion to a separate PR since runtime tunability needs a coordinated wire-level handshake between producer and validator.

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Generated by Claude Code