refactor(cli): switch compare to relayburn_sdk::normalize_since (#421)

* refactor(cli): switch compare to relayburn_sdk::normalize_since

`burn compare` was the only presenter still carrying its own
`normalize_since` + Hinnant civil-date math; the SDK has exposed
`normalize_since` for a while and the sibling `summary` and `hotspots`
presenters already use it.

Delete the local helpers and their tests, drop ~380 LOC of duplicated
date plumbing. The cli-golden snapshot still matches.

Closes #333

https://claude.ai/code/session_01VFvKstujMoXwZkP5vwrHxQ

* fix(sdk): canonicalize normalize_since to UTC .mmmZ

Codex / Devin / coderabbit flagged a latent regression on PR #421:
the SDK's `normalize_since` returned relative ranges as `...SSZ` (no
millis) and passed ISO inputs through unchanged. Because the ledger
filter is `ts >= ?` against `.mmmZ` rows in SQLite (lex-compared),
this silently dropped same-second turns (`.` < `Z`) and misordered
offset-bearing cutoffs (e.g. `-07:00` sorting before any UTC row).

This was already true for `burn summary` and `burn hotspots`; PR #421
just made `burn compare` adopt the same behavior. The fix is in the
SDK so all three presenters get consistent, correct date filtering:

- Relative ranges emit canonical `YYYY-MM-DDTHH:MM:SS.000Z` (24 chars).
- ISO inputs parse and re-emit as UTC `.mmmZ`: offsets get converted
  (`...-07:00` → `...07:00:00.000Z`), no-fraction widens to `.000Z`,
  sub-ms truncates to ms, `Z`/`z` and date-only forms supported.
- Garbage errors out (no more `looks_like_iso` permissive pass-through).

Tests cover each shape and the lex-compare property against ledger rows.
Also rewords the CHANGELOG bullet to be user-impact-first per
coderabbit's note.

https://claude.ai/code/session_01VFvKstujMoXwZkP5vwrHxQ

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: willwashburn

CHANGELOG.md

@@ -4,6 +4,18 @@ Cross-package release notes for relayburn. Package changelogs contain package-le
 
 ## [Unreleased]
 
+### Changed
+
+- `relayburn-cli`: `burn compare --since` now uses the same normalization path
+  as `burn summary` and `burn hotspots`, keeping date-filter behavior
+  consistent across commands.
+- `relayburn-sdk`: `normalize_since` now canonicalizes outputs to UTC
+  `YYYY-MM-DDTHH:MM:SS.mmmZ`. Relative ranges (`7d`, `24h`) emit `.000Z`
+  instead of `Z`, and ISO inputs with offsets (e.g. `...-07:00`) are
+  converted to UTC. This fixes `burn summary` / `burn hotspots` /
+  `burn compare --since` silently dropping same-second turns and misordering
+  offset cutoffs against the ledger's `ts >= ?` lex compare.
+
 ## [2.8.5] - 2026-05-12
 
 ### Changed