chunk format: one `CHUNK_FIELDS` spec, ASCII text in `ChunkSpool`, one BAM tag parser

[jayhesselberth]

Tier P3 · effort M · from the 2026-09-13 review (prep F7, F10, F12)

• The chunk record's field list is spelled four times: chunking/serialization.py:349-477 (iter_chunk_columns, write), :1044-1176 (load_chunks, read as dicts), chunking/table.py:28-47 (_INT_FIELDS/_TEXT_FIELDS, read as columns), and the classifier in tests/test_backend_parity.py. serialization.py:381-382 defaults feature_start/feature_end to literal -5/5 — a third spelling of the rule resolve_feature_window centralised in Rust prepare backend ignores --feature-start: 25-base feature window instead of 20, on every chunk #189 (never hit, since both backends set them, but a trap).
• ChunkSpool keeps every text member as <U in RAM (serialization.py:720-724): read_ids U36 (144 B), sequences_with_kmer_context ~U56 (224 B), sequences U11 (44 B) plus three small ones ≈ 450 B/chunk ≈ 3 GB at 6.7M chunks — the dominant RSS of data prepare now that arrays spill; text_column() (:748-760) then concatenates a second transient copy. table.py:144-165 already reads text as fixed-width S.
• mv/ns/ts are parsed in io/bam_reader.py:151-157 (ReadInfo.__init__) and again in features.py:131-155 (extract_move_table); CL in bam_reader.py:160-166 and preparation/reader.py:277-281.

Acceptance criteria

• One CHUNK_FIELDS spec (field, member, dtype, missing sentinel, is_text) consumed by the writer, the dict loader, ChunkTable and the parity test's classifier; _text() and the -1 sentinels are properties of the spec; the -5/5 literals are gone. Adding a field is one edit plus its parity classification.
• ChunkSpool holds text as fixed-width S and widens to <U blockwise in _write_text_member; the .npz bytes are identical (tests/test_chunk_writer.py); peak RSS of data prepare on the fixture drops (record it).
• MoveTable.from_alignment(aln) is the single tag parser; ReadInfo and extract_move_table delegate; ReadInfo.from_alignment owns CL.
• tests/test_backend_parity.py still fails on an unclassified field.

Non-goals

• The struct-of-arrays Rust return (separate issue), though this spec is what it should target.