feat: Interactive Inspector (#2319)

### What does this PR do?

Adds simple navigation to the inspector, to explore a session starting
from a single command. This creates a new user flow, where a user can
find a history entry in the interactive view (in, say, Global mode), and
hit Ctrl+o to navigate back and forward through that command's session.

IMAGINED USE-CASE: I remembered that I did a sequence of git steps but I
can't remember the order and forgot to document it. I remember that
`reflog` was involved and want to see the actual sequence, and only
those commands.

IMAGINED USE-CASE: I used a curl command to get my IP address for
greenlisting before I connected to the bastion server `abc.xyz` over SSH
- I could easily find the SSH command with abc.xyz, and go back one step
in the session, but without this change, scrolling through all my curl
commands ever run to find a forgotten URL/domain would be too much work.

Since this gives the inspector tab a broader purpose than viewing
analytics, it needs to function even when there are not enough screen
rows for charts -- hence, this PR also introduces an ultracompact mode
for the inspector that _just_ shows the neighbouring history commands
(as simple scrolling three-entry list, with no panes) if there are fewer
than `auto_hide_height` rows (default: 8). Otherwise, the inspector
behaves as normal, except that Up / Down will change the focused command
by navigating through the session. That means there is no "compact" mode
for the inspector - when the interactive search is compact (but not
ultracompact), the inspector shows its usual chart view.

The UX for this could be improved - to keep this PR as lean as it
realistically can be, I have tried to keep the flow very minimal, but a
follow-up PR could introduce some tooltips, nicer ultracompact
formatting, etc.

A minor QoL improvement that comes with this - since I had to deal with
bold text and would otherwise have need a theming exception, I took the
opportunity to ensure the theme engine sets styles completely (so a
theme can have bold), not just colours. To limit scope creep, I do not
add TOML syntax so (for now) you can only customize colours from config
files, but it means that default-bold text (etc.) can now use the
theming engine if the code-defined default Meaning is bolded.

Key changes:

* introduces a simplified inspector tab, with only previous-current-next
commands as rows, in compact mode
* allows navigation through session history within the inspector (so
compact inspector view is still useful)

It also (see comments below):

* makes `compact` into `compactness`, an enum (to better standardize
across inspector/interactive)
* makes the inspector _only_ change layout for ultracompact mode, which
is still compact+(height<8)
* clippy's complexity limit wanted draw split up a little, so not sure
if this is a reasonable minimal way to do so for now
* adds a `(none)` theme to the theming to enable output testing without
styling
* ~~additional tests, although keen for input on how best to do these~~
one functional test, as a starting point
* ~~documentation~~ [minor doc changes
only](https://github.com/atuinsh/docs/pull/72), as I am not sure there
is much to say

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_Was stacked on #2357, which is now in `main`_

## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
- [x] I have checked that there are no existing pull requests for the
same thing

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