Exercise 3: The Adapter Handles a Read Entirely Within One Record Too — Possible Solution
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THE TEST
------------------------------
    heap.insert_record(b'AAAAAAAAAA')   # 10 bytes, record 0, virtual offsets [0, 10)
    heap.insert_record(b'BBBBBBBBBB')   # 10 bytes, record 1, virtual offsets [10, 20)

    vfs.read('/heap2', 2, 4)    # entirely inside record 0
    vfs.read('/heap2', 12, 4)   # entirely inside record 1

RESULT
------------------------------
    read [2:6) (entirely inside record 0): b'AAAA'
    read [12:16) (entirely inside record 1): b'BBBB'

Both reads return exactly the correct 4-byte slice from exactly one
record each, with nothing from the other record mixed in.

WHY THE SAME read() HANDLES BOTH CASES CORRECTLY
------------------------------
read()'s own loop walks the offset index (a list of (start, end,
record_id) tuples) and, for each entry, checks whether the current
read position pos still falls within that record's own [start, end)
range:

    for start, end, record_id in self._offset_index():
        if pos >= end:
            continue
        ...
        chunk = record_data[pos - start : pos - start + remaining]
        ...
        if remaining <= 0:
            break

For the read starting at offset 2 (inside record 0's own [0,10)
range): the loop finds record 0 immediately, extracts exactly 4 bytes
starting at local offset 2, and remaining drops to 0 -- the loop
breaks before ever reaching record 1's own entry at all. For the read
starting at offset 12: the loop's first iteration (record 0, ending at
offset 10) sees pos (12) >= end (10) and skips it entirely via
continue; the second iteration (record 1, [10,20)) is the first one
that actually applies, extracting the correct 4 bytes at local offset
2 within that record.

WHY NO SPECIAL-CASING WAS EVER NEEDED
------------------------------
Finding 4's own cross-record test worked by the SAME mechanism: the
loop simply keeps accumulating bytes across however many index entries
it takes to satisfy remaining, breaking as soon as remaining reaches
0 -- whether that happens on the very first eligible record (this
exercise's own case) or only after crossing into a second one
(Finding 4's own case). There was never a genuinely separate code path
for "single record" versus "spans multiple records" -- both are just
different numbers of loop iterations before the same exit condition
is reached.

WHY THIS WORKS AS AN ANSWER
------------------------------
Testing the single-record case explicitly, rather than assuming it
must obviously work because the harder cross-record case already
passed, confirms the read() method's own general design handles the
easier case not as a lucky side effect, but because the loop's own
logic was never actually specialized to the cross-record scenario in
the first place.