Consolidate reconciliation recency maps into RecencyMap<K>

[folknor]

Right now the monotonic-clock choice is a discipline, not a rule. Every one of
those four sites - ts_last_query, order_local_activity,
position_local_activity, recent_fills_cache - has to remember to stamp from
dst::time::Instant::now() and not self.clock, and to handle a future-marked
instant in the safe direction by hand.

So make it structural. RecencyMap<K> owns Instant::now() internally - you
mark(key), ask within(key, window), prune_older_than(ttl), and there is no
seam left to thread the wrong clock through.

The shape

struct RecencyMap<K> { inner: IndexMap<K, dst::time::Instant> }
mark / contains_key / remove / last_marked
within / within_at / elapsed / elapsed_at    // _at variants take the pass's `now`
prune_older_than

The _at variants exist so the reconciliation loop passes its loop-top now
once instead of resampling Instant::now() per order - the second half of that
same review note. Every checked_duration_since resolves a None
(marked-in-the-future) to the safe direction, matching what each original site
did by hand, so a garbage future instant reads as "0 elapsed, within window"
rather than panicking.

Renames

The ts_*-named fields you flagged in #4376 - ts_* repo-wide means a domain
UnixNanos, but these hold monotonic Instants, so the names lie:

• InflightCheck::ts_submitted -> submitted_at
• InflightCheck::last_query_ts -> last_query_at
• the node scheduler's ts_last_* locals and ReconciliationCheckState fields
  -> last_*_check
• ts_last_query -> order_query_recency (keyed on ClientOrderId, parallels
  order_local_activity; the other three map fields self-document under
  RecencyMap<K>, but this one's ts_ prefix did not survive the type change
  gracefully, so it got a real name too)

Tests

Focused RecencyMap unit tests: mark/contains/remove, monotonic expiry under
paused virtual time, pruning older-than-TTL, and the future-marked-instant safe
direction.

Still to come

The second promised follow-up - close the class: handle_missing_order's
venue-ts_last gate onto receipt time, plus the wider timer/throttle audit.

[cjdsellers]

Hi @folknor,

Thanks for the PR. I like the naming convention here: dropping the Hungarian-notation-esque domain ts_ component when the value is not a UnixNanos and does not represent a domain timestamp makes the clock axis much clearer. Names like submitted_at, last_query_at, and last_*_check read as real-time instants without implying domain time, while RecencyMap keeps the monotonic clock choice structural.

I traced the converted call sites and ran the focused recency, open-order query, inflight, fill-cache, and position-grace test slices. This looks good to me.

Will get to the next PR shortly!