fix(packer): validate blob offsets and bounds in BlobCopier

[Huge]

Problem

In BlobCopier::copy_fast and BlobCopier::copy (crates/core/src/blob/packer.rs), blob slice offsets are computed directly from index locations:

let start = usize::try_from(blob.offset - offset)
    .expect("convert from u32 to usize should not fail!");
let end = usize::try_from(blob.offset + blob.length - offset)
    .expect("convert from u32 to usize should not fail!");
// ...
&data[start..end]

If an index file contains malformed or corrupted metadata where:

1. blob.offset < offset (causing unsigned subtraction underflow / panic),
2. blob.offset + blob.length overflows u32, or
3. start or end exceeds data.len() / read_data.len(),

the process panics with .expect(...) or out-of-bounds slice indexing &data[start..end], crashing any host application performing repo prune, repack, or copy operations.

Fix

• Factored out helper get_blob_slice that uses checked_sub, checked_add, and safe slice lookup data.get(start..end).
• Replaced unchecked conversions and slice indices in copy_fast and copy with calls to get_blob_slice.
• Returns a structured RusticError (ErrorKind::Repository) with diagnostic context (blob_id, pack_id, blob_offset, blob_length, read_offset, read_length) instead of crashing.

Tests

• Added unit test test_get_blob_slice verifying normal slices, base offsets, underflow (blob.offset < offset), overflow (blob.offset + length > u32::MAX), and out-of-bounds access.
• Existing unit tests pass (cargo test -p rustic_core --lib -- blob::packer::tests).
• Integration tests pass (cargo test -p rustic_core --test integration -- prune, cargo test -p rustic_core --test integration -- copy).

Disclaimer: Found the issue by test-driving an LM-powered vulnerability scanner I work on heavily.