Define relation generation contract and mutation instrumentation

[justinjoy]

Parent: #1438. This is the first atomic unit discovered by Architect/Critic review; it must land before cache consumers are switched to generation validation.

Discovery context

Review of #1438 on base origin/issue-1435-mat-cache (f70897f0) found that replacing nrows checks with generation fields without instrumenting every direct mutation would still permit stale materialization, filtered, sorted, and differential views. Rollback can also accidentally restore an old generation number, and COW/replacement can change storage without changing logical rows.

Scope

• Add internal relation identity plus view_generation (logical values/order/schema) and storage_generation (backing-buffer/COW/replacement) to col_rel_t.
• Define checked overflow behavior: no generation wrap may create a false cache hit; saturate/force invalidation or rotate identity.
• Instrument append, direct cell/row writes, permutation/sort, compaction, retraction/rollback restoration, schema/type changes, COW unshare, and replacement-buffer publication exactly once at the correct success boundary.
• Define deep-copy, shared-view, and worker-copy semantics; worker-local mutations must diverge without mutating coordinator state.
• Add focused generation-transition tests, including same-row-count mutation, permutation, rollback, COW, replacement, worker copy, and overflow boundaries.

Explicit exclusions

Do not change materialization/filter/sorted/differential cache lookup rules in this unit; that is the next unit under #1438. Do not add public API/ABI, spill, suspended-reader, or K-Fusion retirement work.

Acceptance criteria

• Every in-scope mutation path advances the correct generation once, including same-row-count changes.
• Storage-only changes advance storage generation without falsely claiming a logical view change.
• Rollback/restoration issues a fresh generation rather than reusing a prior value.
• Deep-copy/shared-view/worker-copy tests prove independent generation behavior.
• Overflow handling cannot produce a false equality with an older generation.
• Focused normal and ASan/UBSan tests, clang-tidy, and existing relation/worker regressions pass.

Dependency

Feeds #1438's cache-consumer generation validation, then #1371/#1372/#1382/#1383.

[justinjoy]

Implementation PR: #1442

The relation identity/view-generation/storage-generation contract and exact-once mutation instrumentation are implemented in the stacked PR. The change covers direct mutation, COW, rollback, sorting, shared-view schema/name/timestamp/graph/compound metadata, allocation-failure transactions, pool rollback, legacy layout preservation, and threading inventory.

Validation: 346 normal tests passed (13 skipped, 0 failed), ASan/UBSan focused/regression 9/9 passed, allocator/pool failure sweeps passed, threading inventory 94/94 passed, optimizer equivalence and clang-tidy passed.

#1442 is stacked on #1435; cache consumers and borrowed-storage lifetime remain in #1384/#1435. This issue is not being closed before the PR is merged.