feat(storage): add DiskManager for raw page I/O

[IndicKangaroo]

Description

Adds DiskManager, the lowest layer of the new storage engine. It does raw
fixed-size page I/O on the .sdb file and nothing else: read a page, write a
page, allocate an empty one. It doesn't know about keys, records or page
contents, and it doesn't call the serializer. Locking stays with the
concurrency manager.

Part of #290. Closes #332 .

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Key Changes

• New src/seriousdb/disk_manager.py: DiskManager with read_page,
  write_page, allocate_page, page_count, sync and close ). Page n is at byte offset n * PAGE_SIZE.
• page_count is read from the file size, so pages added by another handle
  are visible.
• allocate_page always adds a zero-filled page at the end of the file.
  Reusing freed pages is left to the layer that owns the free list.
• Opening a file whose size isn't a multiple of PAGE_SIZE raises. With
  repair_torn_page=True it truncates the partial last page instead, and
  never touches whole pages.
• write_page and allocate_page don't fsync. sync() does, so the WAL
  ordering can be decided later.
• Uses os.pread/os.pwrite where available, with a lock around seek and
  read/write otherwise (Windows).
• New DiskManagerError(ApplicationError) in exceptions.py; OS failures
  still propagate as OSError.
• No changes to existing behavior.

Validation

• uv run pytest tests/test_disk_manager.py (Windows, Python 3.13): 17 passed, 3 skipped.
  The skips are the short read/write and EOF tests, which patch
  os.pread/os.pwrite and so only run on platforms that have them.
• uv run pytest: 2 failed, 227 passed, 3 skipped in 4.80s,
  the failed ones were test_cache.py, which were failing due to implemention of caching before my commits.They are unrelated to this change.
• uv run ruff check . and uv run ruff format --check .: All checks passed!
• uv run ty check .: All checks passed!
• Tests cover round trips and reopen, bounds checking, misaligned files and
  torn-tail repair, short reads and writes, use after close(), and
  concurrent allocate_page() calls on one instance.
• Not tested: the os.pread/os.pwrite path and the short read/write tests
  on the real repo setup, since I ran them on Windows only.

Checklist

• I reviewed the diff and kept it focused on the described change.
• Python changes pass uv run ruff check . and uv run ruff format --check . , or this PR does not change Python.
• Changed behavior is covered by appropriate tests, including a regression test for bug fixes; any limitations are explained under Validation.
• Validation lists the checks or manual flows I ran and their results.
• Relevant documentation is updated, or no documentation change is needed.

[mohammedFawzy0111]

I will have to run the pread and pwrite tests. I will test it on my linux to confirm

[IndicKangaroo]

I will have to run the pread and pwrite tests. I will test it on my linux to confirm

sure, would wait for the results

[VihaanReddyM]

Will check this out soon it's been a busy week.

[mohammedFawzy0111]

I will have to run the pread and pwrite tests. I will test it on my linux to confirm

Sorry for being late I was very busy .
I have ran the tests and they are working as intended.
Platform: Arch linux.

[IndicKangaroo]

I will have to run the pread and pwrite tests. I will test it on my linux to confirm

Sorry for being late I was very busy . I have ran the tests and they are working as intended. Platform: Arch linux.

Awesome, thanks for verifying this on Linux for me! Much appreciated

[IndicKangaroo]

@mattsva and @VihaanReddyM can you please review this pr? i think this is complete..

[VihaanReddyM]

Sorry for such a late review, I have midterms, family issues and a lot of stuff went on in life. I really apologize 🙏

[IndicKangaroo]

Sorry for such a late review, I have midterms, family issues and a lot of stuff went on in life. I really apologize 🙏

No worries at all, take your time, and thanks for the review.

[mattsva]

@IndicKangaroo srry for not review, I did not had the time.

It looks pretty good tho, thanks @VihaanReddyM for merging it 😃