Validate manifest input strictly and report unreadable manifest files cleanly

[Tet-9]

Two fixes in the manifest path, which treats files as untrusted input.

1. Name, glob, sha256, commit and shard checks used re.match with a trailing $, and $ also matches before a trailing newline, so values like "abc\n" passed. They now use fullmatch (manifest.py, sources.py, quantizers, holdout.py).
2. load_manifest let FileNotFoundError, IsADirectoryError and UnicodeDecodeError escape, so excore check on a typo'd path or a binary file printed a raw traceback. They now surface as a ManifestError, which the CLI already reports cleanly with exit code 2.

Tests cover each case.

[ikeige-coder]

Approved. re.match with a $ anchor accepts a trailing newline, so a manifest name like "evil\n" passed validation. Manifest names and patterns come from untrusted miner PRs, so fullmatch is the right fix. Clean errors for unreadable or non-UTF-8 manifests are also a good catch, and the tests cover both. Merging.