sprint: fix Day-7 CI — relax yanked protobuf pin (==5.24.1 -> >=4.25,<6)

[Mark007-R]

What

The Day-7 CI workflow (added in #28) failed on its very first run during `pip install -r requirements.txt`:

```
ERROR: Could not find a version that satisfies the requirement protobuf==5.24.1
(from versions: ..., 5.23.4, 5.26.0, 5.26.1, ...)
ERROR: No matching distribution found for protobuf==5.24.1
```

PyPI yanked the 5.24.x and 5.25.x protobuf releases — available versions now jump from 5.23.4 straight to 5.26.0. The exact pin became unreachable.

Why this only surfaced today

There was no CI before #28. Local dev environments already had `protobuf==5.24.1` installed, so `pip install -r requirements.txt` was a no-op there. The fresh GitHub Actions runner is the first clean install since the upstream yank, which is why CI is the messenger for a pre-existing pin issue.

Fix

Replace the exact pin with a range:

```
protobuf>=4.25,<6
```

`pip --dry-run` on this branch resolves to `protobuf-4.25.9`, which is what streamlit 1.28.1's own pin window (`>=3.20,<5`) allows. No other pin in requirements.txt is affected.

Verification

• Local `pip install --dry-run -r requirements.txt` resolves cleanly to `protobuf-4.25.9`.
• All 31 unit tests still pass on the host with the pre-existing `protobuf 5.24.1` install (no behavioural change).
• Once merged, CI on `dev` should turn green on the next push.

Files touched

• `requirements.txt` — single-line pin change + a 5-line comment explaining the yank.

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