Replace the `MapOffset` tuple with an enum

[CuteChuanChuan]

Is your feature request related to a problem or challenge?

Working on #25272 (issue: #25077 ) raised a question about MapOffset, the resume point of chunked hash map lookups (JoinHashMapType::get_matched_indices_with_limit_offset).

MapOffset is (usize, Option<u64>) and encodes several distinct states by convention:

• (0, None): nothing of this probe batch has been consumed yet
• (row, None): resume at row, from the head of its chain
• (row, Some(next)) with next != 0: resume in the middle of row's chain
• (row, Some(0)): row's chain is finished (0 is the end-of-chain sentinel), so this is the same position as (row + 1, None)

plus the outer None returned when the batch is exhausted.

This causes a few problems:

• Magic-value comparisons. HashJoinStream::process_probe_batch uses state.offset == (0, None) to detect the first chunk of a probe batch. That relies on an invariant that isn't documented: a returned offset is never (0, None), because every path returning (row, None) has already moved past at least one probe row (given limit > 0), while resuming mid-chain on row 0 yields (0, Some(_)).
• Two encodings of the same position, whose meaning depends on the reader. The chain traversal decodes (row, Some(0)) as row + 1, but the unique-key fast paths (join_hash_map.rs, array_map.rs) read only offset.0, so they would treat it as row. They are correct only because they never produce Some.
• Unnamed positional fields, e.g. offset.0 in join_hash_map.rs and array_map.rs.

Describe the solution you'd like

Replace the tuple with an enum, for example:

enum ProbeOffset {
    Start,
    AtRow { row: usize },
    MidChain { row: usize, next: u64 },
}

• (row, Some(0)) becomes AtRow { row: row + 1 }, so each position has a single encoding.
• Keep the outer Option for "batch exhausted" rather than adding a Done variant, since Done is never a valid starting offset.
• state.offset == (0, None) becomes a check for ProbeOffset::Start. This makes the invariant explicit, although it is still upheld by convention rather than by the type.
• If another resume point is added later, the compiler forces every call site to handle it.

Describe alternatives you've considered

• Keep the tuple and document it. Add a doc comment on MapOffset describing the states and the (0, None) invariant. No API change, but correctness still relies on convention, and the two encodings of the same position remain.
• An enum without Start. The first-chunk check becomes AtRow { row: 0 }. This avoids having both Start and AtRow { row: 0 } describe the same position, at the cost of keeping a value comparison.

Additional context

Costs I can see:

• get_matched_indices_with_limit_offset is part of JoinHashMapType, which is public (although documented as mainly intended for internal use), so this would be an API change.
• It touches every implementation (JoinHashMapU32, JoinHashMapU64, PruningJoinHashMap) as well as ArrayMap and traverse_chain.
• Runtime cost should be nil: the offset is built once per lookup call rather than per row, and both representations are 24 bytes on 64-bit targets.

I'm happy to work on this if maintainers think it is worth the API change.