feat(timestamp-stack): add routing timestamp instrumentation

[YuanYuYuan]

Summary

Adds opt-in timestamp instrumentation for measuring end-to-end message latency in Zenoh. Messages can carry a TsStack wire extension that accumulates Interception records at up to three points along a message's path: Send, Route, and Receive.

The feature is entirely #[cfg(feature = "unstable")]-gated and has zero overhead on uninstrumented messages.

Key Changes

Wire protocol (zenoh-protocol, zenoh-codec)

• New TsStackType extension (ID 0x7) added to Push, Request, and Response messages
• WireTimestampStack carries conf_flags (which points are active) + an ordered Vec<Interception>
• Codec enforces a max stack depth of 64 to prevent malformed-wire memory exhaustion
• Round-trip codec tests cover empty stacks, known records, random instances, and the depth-limit boundary

Public API (zenoh crate, feature = "unstable")

• TimestampInstrumentationBuilder / TimestampInstrumentation — configure which points to record
• InterceptionPoint (Send, Route, Receive) — #[non_exhaustive] for forward compatibility
• TimestampStack / TimestampStackRecord — read timestamps off a received Sample, ReplyError, or Query
• TsStackContext / GetTimestampCallback — custom timestamp generation via OpenBuilder::with_timestamp_callback
• .timestamp_instrumentation(Option<TimestampInstrumentation>) builder method on put / get / reply / publisher

Instrumentation points

• Send: session.rs (resolve_put, resolve_get) and builders/reply.rs
• Route: routing/dispatcher/pubsub.rs (per-subscriber, inside fan-out loop) and queries.rs
• Receive: WeakSession::send_push_consume and adminspace.rs

Robustness fixes

• Codec decode cap at 64 interceptions (prevents OOM on crafted wire input)
• WireTimestampStack renamed from TimestampStack to eliminate name collision with the public API type
• get_ts_stack_timestamp removes unwrap()/expect() — returns empty bytes on clock failure instead of panicking
• push_ts_interception uses debug_assert + silent skip instead of expect for unknown point IDs
• ext_ts_stack: None on timeout replies is now documented (was a TODO comment)

Plumbing

• WeakDynamicRuntime / WeakRuntime weak-reference plumbing for runtime access in query state
• ReplyErr public API accessor
• lazy_hlc in RuntimeState — lazily-initialized HLC fallback when no user HLC is configured, replacing the previous SystemTime::now() approach

Tests

• 20 integration tests in zenoh/tests/timestamp_stack.rs: no-instrumentation baseline, single-point (Send / Receive), all-points ordering, custom callback byte verification, callback context correctness, per-message stack independence, query/reply flows, is_custom flag, route-only instrumentation, ReplyError propagation, multiple-subscriber fan-out, and publisher API path
• 4 codec tests: empty stack, known records, random round-trips, and the 64-depth limit

Breaking Changes

None. All new types and methods are behind feature = "unstable".

Known Limitations

• Route timestamp is stamped once per routing hop; multi-hop topologies will produce multiple ROUTE records (by design)
• QueryCleanup timeout responses carry ext_ts_stack: None — not instrumented (documented rationale in code)

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🏷️ Label-Based Checklist

Based on the labels applied to this PR, please complete these additional requirements:

Labels: new feature

🆕 New Feature Requirements

Since this PR adds a new feature:

• Feature scope documented - Clear description of what the feature does and why it's needed
• Minimum necessary code - Implementation is as simple as possible, doesn't overcomplicate the system
• New APIs well-designed - Public APIs are intuitive, consistent with existing APIs
• Comprehensive tests - All functionality is tested (happy path + edge cases + error cases)
• Examples provided - Usage examples in code comments or separate example files
• Documentation added - New docs explaining the feature, its use cases, and API
• Feature flag considered - Entirely behind feature = "unstable", zero overhead when inactive
• Performance impact assessed - No overhead on uninstrumented messages (push_ts_interception is a no-op when ext_ts_stack is None)
• Integration tested - 24/24 integration + codec tests pass (Rust 1.93.0)

Consider: Can this feature be split into smaller, incremental PRs?

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