relayburn-sdk: fix three small Rust correctness bugs

[willwashburn]

Summary

Three small correctness fixes flagged in #337.

1. Fix watch_loop.rs first-tick lag. The non-immediate branch was unconditionally consuming the first interval tick to "skip" an immediate run that branch never produced, so the first periodic tick fired at 2 * interval. Switch to interval_at(now + interval, period) so the first tick lands at ~interval in both modes. Adds a deterministic regression test using start_paused + tokio::time::advance.

2. Rename now_iso → now_lex_token. The function emits ts:NNN.NNN (a sortable lex token), not ISO-8601; the column doc was misleading. Rename + expand doc to call out the format contract; update the matching cutoff helper's pointer in burn state prune. Pure naming/doc fix; no behavior change. Actual ISO-8601 adoption tracked in Rust refactor: collapse 3 hand-rolled civil-date implementations onto the time crate #331.

3. Rename save_cursor_changes → save_cursors_if_changed + fix doc. The doc claimed per-key diffing; the impl is an equality check + full save. Rename and rewrite the doc to match. Per-key delta writes deferred to a separate perf PR.

Closes #337.

Test plan

• cargo build --workspace --all-targets clean
• cargo test --workspace passes (incl. new watch_loop deterministic-timing test)
• cargo clippy --workspace --all-targets no new warnings

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ℹ️ Recent review info

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Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: c9829436-114e-4332-a101-f40863dea4c2

📥 Commits

Reviewing files that changed from the base of the PR and between e5e5b99 and 3b889a7.

📒 Files selected for processing (6)

• crates/relayburn-cli/src/commands/state.rs
• crates/relayburn-sdk/src/ingest.rs
• crates/relayburn-sdk/src/ingest/cursors.rs
• crates/relayburn-sdk/src/ingest/ingest.rs
• crates/relayburn-sdk/src/ingest/watch_loop.rs
• crates/relayburn-sdk/src/ledger/writer.rs

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📝 Walkthrough

Walkthrough

This PR fixes three correctness bugs from issue #337. It standardizes ledger timestamps to a lexicographically sortable format, renames the cursor persistence function to match its actual semantics, and corrects watch loop periodic tick scheduling for non-immediate configurations.

Changes

Rust Correctness Fixes: Timestamp Format, Cursor Semantics, Watch Loop Timing

Layer / File(s)	Summary
Ledger Timestamp Standardization crates/relayburn-sdk/src/ledger/writer.rs, crates/relayburn-cli/src/commands/state.rs	now_iso() is replaced by now_lex_token() to reflect the actual ts:{secs:020}.{nanos:09} format. All call sites in append_stamp, append_content, and debug_now are updated. Documentation in the CLI state command is corrected to reference the lexicographic format.
Cursor Persistence Semantics crates/relayburn-sdk/src/ingest/cursors.rs, crates/relayburn-sdk/src/ingest.rs, crates/relayburn-sdk/src/ingest/ingest.rs	save_cursor_changes() is renamed to save_cursors_if_changed() and documentation is corrected: the function skips writes only when cursors are equal, then persists the entire cursor map (not per-key diffs). Public re-exports and all call sites across five ingest functions (ingest_all, ingest_claude_projects, ingest_codex_sessions, ingest_opencode_sessions, ingest_claude_session) are updated.
Watch Loop First Tick Timing crates/relayburn-sdk/src/ingest/watch_loop.rs	interval() with manual skip is replaced by interval_at(now + interval, interval) to ensure the first periodic tick for non-immediate configurations fires after one interval (not two). A regression test non_immediate_first_tick_lands_at_interval verifies correct timing.

Possibly related PRs

• AgentWorkforce/burn#313: Updates crates/relayburn-cli/src/commands/state.rs for timestamp formatting and state CLI behavior.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 Three bugs scurry through the code,
Timestamps now in sortable mode,
Cursors saved with honest names,
Ticks that fire on rightful frames,
Hop hop, correctness is our shame!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically summarizes the main change: three small Rust correctness bugs related to watch_loop timing, timestamp naming, and cursor persistence function naming.
Description check	✅ Passed	The description is well-detailed and directly related to the changeset. It explains all three fixes, references issue #337, provides test confirmation, and uses clear structured sections.
Linked Issues check	✅ Passed	The PR fully addresses all three coding requirements from issue #337: fixes watch_loop first-tick lag using interval_at(), renames now_iso to now_lex_token with updated docs, and renames save_cursor_changes to save_cursors_if_changed with corrected documentation.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to the three bugs identified in issue #337. The modifications to state.rs, ingest.rs, cursors.rs, ingest.rs, watch_loop.rs, and writer.rs are all necessary implementations of the fixes with no extraneous changes detected.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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