Rename `registryStatus()` to `getRegistryStatus()` for module-root naming consistency

[cb1kenobi]

Summary

Rename the module-root export registryStatus() to getRegistryStatus(), keeping
registryStatus() as a deprecated alias until the next major (v3.0.0) removes it.

Why

The module root has two conventions for the same shape of API — a zero-argument function
that samples live native state and returns a plain object:

Export	Convention
registryStatus()	bare noun
getWriteBufferManagerStats() (added in #824)	get prefix

Both walk the database registry and return a snapshot. There is no principled reason for
them to read differently, and whichever one we leave alone becomes the precedent for the
next diagnostic export.

get wins because these calls do real work rather than reading a stored property.
getWriteBufferManagerStats() takes a try_lock on the registry and can return
inventoryAvailable: false; registryStatus() walks every descriptor. The codebase already
reserves bare nouns for cheap, stable properties (db.compression, db.status) and uses
get* for the sampling calls (db.getStats(), db.getStat(), db.getEstimatedKeyCount()).
Renaming the one non-conforming export is a smaller change than converting the new one to a
getter, and it does not put a lock-taking call behind property-access syntax.

Proposed change

• Add getRegistryStatus() as the canonical name.
• Keep registryStatus() exported as an alias to the same function, marked
  @deprecated in JSDoc, pointing at the new name.
• Remove the alias in v3.0.0.
• README documents getRegistryStatus() only. The deprecated alias does not need a
  README entry; the JSDoc @deprecated tag is the discovery path for existing callers.

Scope

• src/load-binding.ts — export getRegistryStatus, retain registryStatus as a
  deprecated alias with a JSDoc @deprecated tag naming the replacement and the
  removal version.
• src/index.ts — re-export both.
• README.md — rename the registryStatus(): RegistryStatus section heading and its
  example, and update any anchor links to it.
• Tests — add coverage asserting the alias and the new name resolve to the same
  function, so the deprecation window is enforced rather than assumed.

Downstream consumers

registryStatus() is used in both Harper repos today, so the alias is what keeps this
non-breaking:

• harper — dataLayer/rocksdbBackup.ts (production path), plus three files under
  unitTests/resources/
• harper-pro — core/dataLayer/rocksdbBackup.ts (production path), plus one file under
  core/unitTests/resources/

Migrating those call sites is a follow-up and does not gate this change. They must be
migrated before the alias is removed in v3.0.0.

Context

Came out of review on #824 (Make a WriteBufferManager write stall observable), which
introduced getWriteBufferManagerStats(). That PR originally exposed the function twice, as
a RocksDatabase static and as a module-root export; the static is being dropped in favor of
the top-level export, which is what puts the two naming conventions side by side at the
module root.