Gantry: 11 - negcache callback lock hygiene

## Scope

Move negcache metric callbacks outside the mutex.

## Problem

negcache metric callbacks are invoked while holding `c.mu`. Current callbacks are safe, but future callbacks that re-enter the cache can deadlock.

Evidence:

- `internal/gantry/negcache/negcache.go:117-122`

## Changes

- Snapshot callback data under lock, unlock, then invoke `OnEnter`, `OnHit`, and `OnSize`.

## Tests

- A negcache callback that calls back into `Len()` does not deadlock.

Start in internal/gantry/negcache/negcache.go at lines 117-122 and trace the negcache metric callbacks. Verify that callback data is snapshotted while c.mu is held, then that OnEnter, OnHit, and OnSize run after unlocking. Add or run a test where a callback calls Len() and confirm it does not deadlock.

Written by the indexing model from the issue text.