feat(node): read-only GET /v1/usage endpoint (Phase P2)

[varshith-Git]

Adds usage accounting for Valori Cloud's plan/quota system — records count, collection count, and billable storage bytes (event-log + archived rotation segments), read directly from the live engine. Never mutates state (proven by a dedicated test comparing the BLAKE3 state hash before/after a call).

• server.rs: usage_handler() (standalone), storage_bytes_standalone(), file_size(); sum_log_and_archives() made pub(crate) for reuse.
• cluster_server.rs: usage() handler sums records/storage across all shards; collections read from shard 0's namespace registry (correct, since the registry itself isn't sharded).
• tests/usage_endpoint_tests.rs: 5 tests covering fresh-engine defaults, real inserts, collection create/drop, rotated-segment storage accounting, and the never-mutates-state invariant.

Both standalone and cluster paths implemented per the mandatory dual-path checklist (CLAUDE.md).

Summary

What does this change do, and why?

Determinism Impact

• No determinism impact
• Improves determinism
• Changes arithmetic behavior (Q16.16 / FxpScalar)

If arithmetic behavior changes, explain why and what compatibility fixtures were updated.

no_std / Kernel ABI

Only applies if crates/valori-kernel or crates/valori-core were touched.

• N/A — kernel/core not touched
• No use std:: added inside valori-kernel/src/; std-only code is gated behind #[cfg(feature = "std")]
• cargo build -p valori-kernel --target wasm32-unknown-unknown passes locally

Standalone + Cluster Parity

Only applies if this adds or changes an HTTP endpoint or a KernelEvent.

• N/A — no endpoint/event change
• Handler added to both server.rs and cluster_server.rs
• Write path in cluster mode goes through raft.client_write(), not a direct engine lock
• cargo test -p valori-node --test route_parity passes (or the allowlist was updated with a reason)

Cross-Architecture Behavior

Tested on:

• x86
• ARM / Apple Silicon
• Embedded / Jetson

Tests

• cargo test -p valori-kernel -p valori-node passes
• cargo fmt --all -- --check and cargo clippy --workspace --all-targets -- -D warnings pass

Include reproduction / validation cases below.

UI (if applicable)

• N/A — no UI change
• Verified in both dark and light mode (semantic tokens only, no hardcoded colors)

Notes

If this relates to forensic or evaluator tracks, confirm that logic is kernel-appropriate.

Anything deferred to a follow-up, and which phase/issue owns it.