Draft multiply indexed filesystem link graph

[isomorphisms]

Purpose

Exercise the fragment/link/index storage idea entirely on the Grease side, with ordinary files and symlinks and no Idriç dependency, against the access shape described by IB PR #52.

Semantic boundary

This draft is testing four roles:

• durable source/link records with stable fragment identities;
• rebuildable source- and destination-oriented query projections;
• batched graph input rather than one process/table scan per edge;
• a materialized ordered strand that the reader can consume without graph hops.

The current names root-cellar/, fragments/, cauldron/, pensive/strands/, from/, and to/ are prototype spellings, not an architecture or compatibility contract. A later representation may rename, collapse, or compile them while preserving the access semantics.

Current prototype

• links.tsv is the current durable logical graph: source fragment, relation, destination fragment.
• every graph edge gets rebuildable forward and reverse symlink projections;
• from and to enumerate those generated endpoint projections instead of rescanning the complete link table;
• links ROOT accepts a batch of tab-separated edges and rebuilds once;
• indexes.tsv describes additional caller-defined projections;
• strand reads the logical link table once, follows one relation in memory, and writes the selected ordered fragment ids to stdout;
• the caller chooses where a materialized strand is stored;
• rebuild destroys and reconstructs generated projections from the durable tables.

The replacement path also avoids plain mv temporary existing-symlink-to-directory, which GNU mv interprets as a directory destination rather than replacing the projection.

IB-shaped exercise

tools/linkfs/ib-shape-acceptance.sh uses:

• 4,096 document-order fragments, matching the current IB prepaint block ceiling;
• 64 sparse citation edges and 15 annotation edges, for 4,174 logical edges total;
• source and reverse-destination queries through the generated projections;
• one full materialized document-order strand;
• the 64-fragment window used by the IB fragment-link design note;
• deletion of the graph/projection state after strand construction, followed by another successful strand/window read.

For the current literal projection layout the receipt observes 8,348 symlinks and 16,514 directories. It reports filesystem KiB and strand bytes without making runner-specific storage costs a pass/fail threshold.

Finding

The prototype now supports the important semantic split in IB #52:

• filesystem projections work as rebuildable shell/query indexes in both directions;
• strand construction avoids a graph lookup per hop;
• a materialized strand remains independently sequentially consumable.

It does not establish that literal directory/symlink projections belong on the Android reader hot path. The inode/path amplification is already large at the 4,096-fragment shape, and the current newline strand does not yet prove a final mmap format or constant-time positional range lookup. Compact source/destination adjacency and an indexed/range-readable strand representation remain separate implementation work.

Keep this PR draft while those boundaries are being measured; do not promote the present directory vocabulary into permanent IB architecture.

Acceptance

The workflow runs two receipts:

• tools/linkfs/acceptance.sh for small correctness, rebuild, replacement, and non-destructive initialization;
• tools/linkfs/ib-shape-acceptance.sh for the IB-sized topology and strand hot-path boundary.

Both pass on the current PR head. The workflow does not bootstrap or checkout Idriç.