add performance tips page; java BatchUpdater coverage

[brharrington]

Net-new docs/spectator/core/performance.md collects the cross-language patterns that previously lived in a small note on core/meters/counter or were unwritten: cache the meter reference outside hot paths, build ids with createId + withTag for dynamic dimensions, pre-aggregate in tight loops, prefer basic meters over percentile variants, keep tag cardinality bounded. Adds a Java-specific subsection on BatchUpdater (Counter/Timer/DistributionSummary expose a batchUpdater(batchSize) method that buffers updates and flushes on close).

Each Java meter page (counter, timer, dist-summary) gets a "Batch Updates" section; the counter page carries the annotated example and timer/dist-summary cross-reference it.

Removed the now-redundant high-perf-tight-loop note from core/meters/ counter.md and replaced it with a link to the new page.

Also fixed two stale links left over from the percentile merge:

• java/meters/timer.md Percentile Timer section: Timer ->
  Timer (self-link).
• java/meters/dist-summary.md Percentile Distribution Summary section: had an orphan "concept and storage-cost caveats" fragment from where my merge script clipped a multi-line backlink. Rewrote to a single "Use a regular Distribution Summary unless..." sentence linking the in-page anchor.

Nav: Performance Tips added under Core Concepts.