Trim comments across Rust and webapp source to direct one-liners (ticket 030)

Pass over src/*.rs, webapp/src/**, and the in-app "?" docs prose:
condensed multi-paragraph doc comments to single lines that state the
non-obvious why, deleted comments that only restated adjacent code, and
tightened the Docs.tsx help-panel wording. No logic, structure, or code
formatting changed — verified via cargo test (143 passed) and
tsc -b && vite build.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/index_key.rs

@@ -3,11 +3,7 @@ use crate::timestamp::Timestamp;
 use serde::Serialize;
 use std::collections::BTreeMap;
 
-/// Key type for the engine's primary index.
-///
-/// Variants are ordered top to bottom as declared (derived `Ord` compares the
-/// discriminant first), which is what carves the map into contiguous ranges:
-/// logs, then params, then metric declarations, then buckets.
+/// Key type for the engine's primary index. Variant order matters: derived `Ord` compares the discriminant first, carving the map into contiguous ranges in declaration order (logs, params, metric declarations, buckets).
 #[allow(dead_code)]
 #[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Serialize)]
 pub enum IndexKey {
@@ -33,13 +29,7 @@ pub enum IndexKey {
         metric_name: String,
         labels: BTreeMap<String, MetricPrimitive>,
     },
-    /// Ticket 023: internal bookkeeping (never returned by a query) tracking
-    /// the timestamp of the most recent counter event for one metric+label
-    /// combination, independent of whether that event actually pushed a
-    /// `Bucket` value — a `Duration`-kind `Value` param's *first* occurrence
-    /// of a label set has no previous event to diff against and pushes no
-    /// `Bucket` entry, but must still be recorded here so the *next*
-    /// occurrence can compute its elapsed time.
+    /// Internal bookkeeping tracking the last event timestamp per metric+label, so a `Duration` param's first occurrence (which pushes no `Bucket`) still lets the next occurrence compute elapsed time.
     LastMetricEvent {
         metric_name: String,
         labels: BTreeMap<String, MetricPrimitive>,