Repository structure modernization

We'd like to discuss two friction points in the repo layout, both about the public include surface. Filing as a discussion because any change here would break the public API and warrant a major version bump; better to agree on the destination before anyone writes a patch.

## Problem 1: Generic top-level header names collide across libraries

Public headers sit at shallow paths with generic filenames, installed flat under a single directory:

```
hll/include/hll.hpp
theta/include/theta_sketch.hpp
common/include/serde.hpp
common/include/optional.hpp
# ...installed via `DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/DataSketches"`
```

Consumers write `#include `. Nothing in the path identifies the library, and names like `hll.hpp`, `serde.hpp`, `optional.hpp`, and `tdigest.hpp` are plausible choices for any number of unrelated projects. A consumer linking against datasketches alongside another library that also ships an `hll.hpp` ends up with two headers competing for the same include path, with no clean way to disambiguate.

The fix is to namespace the include path under the library name. Two shapes are worth considering.

### Option A: Nested per-sketch directories, drop the filename prefix

```
include/datasketches/
hll/
sketch.hpp # was hll.hpp
detail/...
theta/
sketch.hpp # was theta_sketch.hpp
union.hpp # was theta_union.hpp
intersection.hpp
a_not_b.hpp
jaccard_similarity.hpp
detail/...
cpc/
sketch.hpp
detail/...
...
```

Consumer: `#include `

The directory does the namespacing work; the filename drops the now-redundant `theta_` prefix. Matches Boost (``).

### Option B: Flat namespace, keep descriptive filenames

```
include/datasketches/
hll_sketch.hpp # was hll.hpp
theta_sketch.hpp
theta_union.hpp
theta_intersection.hpp
theta_a_not_b.hpp
theta_jaccard_similarity.hpp
cpc_sketch.hpp
...
detail/
hll/...
theta/...
```

Consumer: `#include `

The filename prefix carries the grouping, and the public surface stays on a single flat level. Matches Abseil (``).

Either way, the install path becomes `${CMAKE_INSTALL_INCLUDEDIR}/datasketches/`.

## Problem 2: Public and internal headers live in the same directory

`hll/include/` mixes the public `hll.hpp` with roughly 30 implementation headers (e.g. `HllSketchImpl.hpp`, `AuxHashMap.hpp`) plus another 15-odd `*-internal.hpp` files. All of them get installed by `hll/CMakeLists.txt`. From a consumer standpoint, this seems valid:

```cpp
#include
#include
```

There is no clear API boundary between public and internal headers.

The fix is a per-sketch `detail/` subdirectory, a convention Boost has used since the early 2000s and that Abseil mirrors with its `internal/` directories. Implementation headers move there, and the contract for consumers is straightforward: anything inside `detail/` is internal, and reaching in is at your own risk.

## Low-priority items

- **Header guards.** Two styles coexist: `hll/include/HllArray.hpp` uses `_HLLARRAY_HPP_` (leading underscore, reserved by the standard); `theta/include/theta_sketch.hpp` uses `THETA_SKETCH_HPP_`. Switching all headers to `#pragma once` removes the inconsistency, the reserved-identifier risk, and a class of copy-paste mistakes.
- **No `.clang-format`.** Style is enforced by convention only. Committing a single file would replace formatting back-and-forth in code review.
- **Pick one casing convention.** File names and code currently mix CamelCase and snake_case. Low-priority but easy on the eyes.
- **Tests scattered per-sketch.** Each `/test/CMakeLists.txt` repeats the Catch2 wiring and test-discovery setup. Consolidating into a single `test/` tree (with per-sketch subdirs) would declare the test framework once, give shared fixtures (random key generators, serde helpers) an obvious home. The current setup works; this is cleanup, not a fix.

Start by reading hll/CMakeLists.txt and the per-sketch include and test CMakeLists files mentioned in the issue. Trace which headers are installed and how test discovery is repeated, then compare the proposed include layouts and detail directories with current consumers. Done means the project agrees on a destination and migration scope before implementation begins.

Written by the indexing model from the issue text.