docs(ssi): retract overclaimed serializability guarantees

[gburd]

Outside review (#136–#140) showed the SSI docs describe the intended design, not the delivered behavior. Correcting them now, rather than after the fixes, so we are not shipping a promise we don't keep.

• rfc/0003 — drop "the commit-time pivot check is race-free against concurrent edge recording" and "markers are reclaimed incrementally and bounded". Add a Known limitations section naming libdb 5.3.34: DB_TXN_SNAPSHOT commits a write skew when the second write starts during the first commit #136 (write skew can commit when a conflicting write lands inside DB_TXN->commit, plus the separate different-pages-of-one-btree observation), libdb 5.3.34: sequential read-only DB_TXN_SNAPSHOT transactions eventually cause DB_ENV->txn_begin to return ENOMEM #137/libdb 5.3.34: __txn_reap_si_details leaks MVCC mutex slots during snapshot cleanup #138 (reader bookkeeping not fully reclaimed → mutex-region exhaustion), and libdb 5.3.34: incomplete replication commit lock lists can violate client transaction isolation #140 (DB_LOCK_SIREAD unaccounted in the replication commit lock-list sizing). The section is removed only when each item is fixed and covered by a test.
• docs_src/api/c/txnbegin.md — user-facing Known-limitation note under DB_TXN_SNAPSHOT, so applications don't rely on serializability alone yet.

No engine change. The five fixes are in flight on separate branches; this is the honesty fix that shouldn't wait for them.

Related: #136, #137, #138, #139, #140

[github-actions]

Coccinelle convention checks

No new violations. ✅

Resolved since baseline (2) -- update dist/cocci/baseline.txt to lock these in.

rule_mutex_unbalanced|MUTEX_UNBALANCED|src/crypto/mersenne/mt19937db.c|return (ret);
rule_mutex_unbalanced|MUTEX_UNBALANCED|src/mp/mp_register.c|return (ret);

[github-actions]

ABI diff vs v5.3.34 (libabigail — authoritative)

Removed exported symbols (nm -D, _NNNN version suffix normalized)

None.

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Advisory: libabigail/nm is the authoritative binary-ABI check; Coccinelle is complementary source-level early warning. See dist/cocci/README.md.