Split monolithic index.c into focused components:
- src/am/ - access method (handler, build, scan, vacuum)
- src/types/ - data types (vector, query, score)
- src/state/ - index state management (state, registry, metapage, limit)
- src/planner/ - query planner hooks and cost estimation
- src/debug/ - debugging utilities (dump)
- src/segment/ - disk segment operations (renamed merge.c, query.c)

Also includes:
- Move TpPostingEntry/TpPostingList structs to posting.h
- Add segment/pagemapper.h for address translation utilities

🤖 Generated with [Claude Code](https://claude.com/claude-code)

- Add <access/htup_details.h> for GETSTRUCT macro
- Remove excess elements from relopt_parse_elt initializers

Introduces a columnar interface that both memtable and segment can
implement, allowing scoring code to be storage-agnostic:

- src/source.h: TpPostingData (columnar ctids/frequencies) and
  TpDataSource interface with get_postings, get_doc_length, close
- src/source.c: Helper functions for allocating/freeing posting data
- src/memtable/source.{h,c}: Memtable implementation of TpDataSource

This is the foundation for decoupling memtable from segment knowledge.

Implements the columnar TpDataSource interface for segments:

- Supports V2 block-based segment format with skip indices
- Converts block postings to columnar format with CTIDs
- Uses cached CTID arrays for doc_id to CTID translation
- Dequantizes fieldnorm for document length lookups

This completes the TpDataSource abstraction layer, allowing scoring
code to work uniformly with both memtable and segment data.

Replace direct memtable internal access (string_table, doclength_table,
TpPostingList, TpPostingEntry) with the abstract TpDataSource interface.
This makes the scoring code independent of memtable implementation
details and uses the new columnar TpPostingData format.

The segment scoring continues to use tp_score_all_terms_in_segment_chain
which has important optimizations for processing all terms in a single
segment traversal.

Dead code removed:
- tp_destroy_index_dsa, tp_registry_detach_dsa, tp_registry_reset_dsa,
  tp_registry_get_shared_dp (unimplemented declarations)
- tp_release_local_index_state, tp_get_local_index_state_if_cached,
  tp_destroy_shared_index_state (unused implementations in state.c)
- tp_recover_from_docid_pages (~170 lines, unused in metapage.c)
- tp_string_table_delete, tp_string_table_get_or_create_posting_list,
  tp_realloc_posting_entries_dsa, tp_get_string_from_dp (stringtable)

Bug fix:
- calculate_term_score() was using hardcoded k1=1.2, b=0.75 instead of
  the actual index parameters from metapage. Now correctly passes
  metap->k1 and metap->b to the scoring function.

Total: 376 lines removed.