chore: add .gitattributes -- no line-ending normalization, CRLF and mixed endings already committed

## Problem

This repository has **no `.gitattributes`**, so git applies no line-ending normalization. Whatever line endings a tool happens to write are what get committed, and they differ per machine and per editor.

Measured with `git ls-files --eol` on the current index:

| | files |
|---|---|
| LF | 29 |
| **CRLF** | **54** |
| **mixed (both inside one file)** | **4** |

Mixed-ending files are the worst of these: a single file containing both endings produces diffs where lines appear changed although only the invisible ending differs, and they make `git blame` unreliable.

## Why it happens here

`core.autocrlf` is `false` at the **system** level (set by the Git-for-Windows install, "checkout as-is, commit as-is") and unset globally. With no `.gitattributes` either, nothing normalizes anything on the way into the object database.

## Recommended fix

**1. Add a `.gitattributes` at the repository root.** This is the authoritative mechanism: it lives in the repo, travels to every clone, and applies to every contributor and to CI — unlike `core.autocrlf`, which is per-machine and invisible to everyone else. It also takes precedence over `core.autocrlf`.

A reasonable starting point:

```gitattributes
# Normalize all text files to LF in the object database.
* text=auto eol=lf

# Files that must stay LF to run on Linux/macOS.
*.sh text eol=lf
```

Add `*.bat`/`*.cmd` as `text eol=crlf` if this repo ships any — those genuinely need CRLF on Windows. Adjust the rest to the languages actually present.

**2. Then renormalize, as a separate commit.**

```bash
git add --renormalize .
git commit -m "chore: normalize line endings"
```

Order matters: `.gitattributes` first, then renormalize. Renormalizing first just re-commits whatever the local `core.autocrlf` decides, and the work is repeated later.

**3. Time it deliberately.** The renormalize commit touches every affected file, so do it when the repo is quiet — it will conflict with anything in flight, and it makes a large, noisy diff that reviewers should be told to skip.

## Note

A machine-wide `git config --global core.autocrlf input` is a useful safety net for repos that lack `.gitattributes`, but it is **not** a substitute: it only protects the machine that sets it, so a collaborator or CI runner without it still commits CRLF. The `.gitattributes` file is what actually fixes this for everyone.

Found while auditing line endings across the local repository set.

Start at the repository root and inspect the current file endings with `git ls-files --eol`. Add `.gitattributes` using the issue's proposed rules, then create the renormalization as a separate commit with `git add --renormalize .`; done means the attributes file is committed first and the resulting line-ending changes are isolated for review.

Written by the indexing model from the issue text.