Enforce SVN fingerprint byte cap during reads

[lzl51230]

Related to #1113.

SVN fingerprinting checks the file size before opening, but then reads until EOF. A file that grows concurrently can exceed the configured fingerprint byte cap and consume unbounded watcher-cycle time and I/O.

Track cumulative bytes while reading, stop at the existing cap, and validate size from the opened handle where possible. Add coverage for a file that grows after the initial metadata check.

[DeusData]

Tracked as a 0.9.2-rc stability blocker for #1113/#1127. The byte cap must be enforced against bytes actually read from the opened handle, not only pre-open metadata. The RED test should grow the file after the initial check and prove both bounded I/O and deterministic fingerprint behavior at the cap.