[NIDX-01 workstream] Replace bounded read-only index access

Parent: #13990

Tracking parent — do not apply Backlog. The TDD workflow selects leaf issues only. This workstream closes after all five ordered children merge.

Why this workstream is split

A single issue previously combined generation open, segment count, deletion masks, stored chunks, dictionaries/postings, doc values, search-after, fixtures, and six production callers. That requires roughly the complete ICE read path and cannot freeze one behavioral seam plus RED/e2e tests in one TDD contract turn.

The children are sliced by fixture complexity against live callers, not by dormant format layers. Every child:

• adds at most one new ICE read capability;
• switches a named production caller in the same merge;
• owns independently generated compatibility-writer fixture bytes and literal expected values;
• exposes one BanyanDB-owned behavioral seam;
• writes no index bytes; and
• is independently reversible without converting a directory.

End-state boundary

After all children merge, these direct Bluge reads are native:

• inverted.ReadOnlyDocCount;
• Measure and Stream migration verification counts;
• Measure index-mode copy and series-union source walks;
• metadata/schema/reader.WalkShard and WalkDocs; and
• Property repair tuple pagination/tree construction.

The complete bounded surface is committed-generation open, visible count, match-all stored walk, exact term/OR, deletion masks, required scalar decoding, and ascending Property repair search-after. Writers, arbitrary queries, and lifecycle behavior remain outside NIDX-01.

Ordered TDD leaves

1. #14008 — NIDX-01A: baseline single-segment read-only count
2. #14009 — NIDX-01B: committed generations and deletion masks, blocked by [NIDX-01A] Cut over baseline read-only document count #14008
3. #14010 — NIDX-01C: stored document walks for migration sources, blocked by [NIDX-01B] Count committed generations with deletions #14009
4. #14011 — NIDX-01D: exact-term schema walks, blocked by [NIDX-01C] Walk stored documents for migration source reads #14010
5. #14012 — NIDX-01E: Property repair search-after, blocked by [NIDX-01D] Cut over exact-term schema walks #14011

Only the oldest unblocked open leaf may carry Backlog. Do not label this parent or multiple leaves. An open pull request does not unblock its successor; the preceding leaf must merge first.

Completion criteria

• All five children are merged and closed.
• Every named production caller uses the native reader.
• Historical fixture, corruption, concurrency, bounds, and immutability tests pass.
• Source checks prove every named caller reaches only the native read boundary.
• No child merged a test-only parser, unused native API, or format-only foundation.

Lexical non-regression

Repository changes for every implementation leaf before the final removal may delete existing references but must add zero new case-insensitive bluge tokens and zero matching tracked paths. The gate includes imports and aliases, function/type/variable names, filenames and runtime names, strings, comments and messages, tests, fixture/provenance data, scripts, configuration, and generated assets. Compatibility evidence uses neutral legacy oracle or compatibility writer labels plus an immutable revision or content hash rather than adding a retired module name.

This lexical gate applies to repository changes, not to issue or archived-design prose that names the dependency in order to specify its removal.

Design

BDB-NIDX-SPEC-001 revision 0.2 — NIDX-01 behavior