docs(alter): document SET SORTED BY ASC/DESC and CSR

[adsharma]

Documents SET SORTED BY ASC/DESC + CSR in src/content/docs/cypher/data-definition/alter.md for LadybugDB/ladybug#782 and #799.

• Node non-CSR ASC/DESC matrix: any existing column(s), ASC/DESC, single or composite (mixed directions allowed); metadata only — no ordering enforced, no watermark, no optimizer effect. New declaration replaces prior metadata; plain form clears CSR flag; RENAME COLUMN follows renames; unknown columns rejected; visible in EXPLAIN.
• Node CSR (#782): exactly one property, ASC, must be the primary key (single ascending PK + CSR). DESC, composite, or non-PK with CSR rejected. User assertion (primary_key == rowid, csr_index interchangeable with table_offset); not verified from data. Sets CSR flag + change-epoch watermark; mutation invalidates and optimizer silently disregards flag. Enables offset-count degree rewrite and COUNT(DISTINCT nbr) bounded per-depth BFS rewrite when conditions met.
• Rel CSR (#799): CSR keyword required on rel tables; only (FROM ASC, TO ASC) accepted (FROM/TO are structural endpoint names, not properties). User assertion — storage does not sort at checkpoint; declare only on pre-sorted data. Lets CSRArrowArrays::symmetrize() skip per-row sort; csrChangeEpoch watermark recorded; post-declaration mutation falls back to safe per-row sort.
• Validation + invalidation semantics: binder exceptions for each rejected shape; watermark/invalidation behavior for both node and rel CSR.
• Verification: npm run build passes.

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