fix: Dramatically decrease daemon memory usage (#3211)

Reduces memory usage of the atuin-daemon search index by ~80% through
string interning, compact UUID storage, and eliminating redundant data.

## Changes

* **Eliminate Vec\<Invocation\>**: Replaced the per-command
`Vec<Invocation>` with just `most_recent_id: [u8; 16]` and
`most_recent_timestamp: i64`. We only ever needed the most recent
history ID for search results - the full invocation history was never
used.
* **UUID byte storage**: Store UUIDs as `[u8; 16]` instead of 36-byte
strings, saving 40 bytes per UUID.
* **String interning with lasso**: Use `ThreadedRodeo` to deduplicate
`cwd` and `hostname` strings in the filter sets. These values are highly
repetitive (most commands run from a small set of directories on the
same host), so interning has an outsized effect.
* **DashSet → HashSet**: Since `CommandData` lives inside a `DashMap`
(already synchronized), the inner sets don't need their own locks.
Switched to `HashSet<Spur>` for directories/hosts and `HashSet<[u8;
16]>` for sessions.
* **Arc\<str\> for commands**: Changed the `commands` DashMap key and
`frecency_map` keys from `String` to `Arc<str>`, enabling zero-copy
sharing between the two maps.
* **Remove dead code**: Removed `CommandData.command` field that was
duplicating the DashMap key.

## Results

With 60k history entries, observed memory usage dropped from ~200MB to
~40MB.

0 files changed, 0 insertions, 0 deletions